Kelly was drafted three times: – In 2007 by the Orioles in the 37th round, out of Desert Mountain High School in Scottsdale, Arizona.– In 2009 by the Indians in the 22nd round, out of Yavapai Junior College in Arizona.– In 2010 by the Rays in the 8th round, out of Arizona State. This time he signed for a bonus of $125,000, via scout Jayson Durocher.

Merrill's dad, Tom Kelly, was the GM of the Chicago Ritz-Carlton and others within the Four Seasons Hotel corporation over 35 years before taking early retirement.

Merrill's brother, Reid, played five Minor League seasons in the Astros (2006-2009) and White Sox (2010) organizations.

Merrill is a strong competitor. And he enjoyed pitching in Korea. “It’s an interesting ballgame, it’s a completely different animal from what we see in the states," said Kelly. "The balls are a little smaller, the parks for the most part are a little smaller. The fans over there are crazy, they’re baseball crazy. They cheer from pitch 1 to pitch 250 of the game. It doesn’t matter if their home team is up by 10 or losing by 10, they’re in it from the start until the last out in the 9th. It can be intimidating when you first get over there, but the longer I was over there I learned to love it and the atmosphere was actually really cool." 

Merrill had never pitched in the Major Leagues, but he posted decent numbers in the Rays' minor league system from 2010-2014 before becoming a durable four-year starter for the SK Wyverns in the hitter-friendly Korea Baseball Organization.

In 2019, the 30-year-old righthander should have a spot in the Arizona rotation to open the season, assuming all goes well in 2019 spring training.

Kelly’s ascent to the Major Leagues is unprecedented. Never before has a player with zero Major League experience played in Korea and returned directly to MLB. In 2016, reliever Tony Barnette came back from Japan to make his big-league debut at 32 with the Texas Rangers. And Eric Thames—who, like Kelly, is represented by Sosnick, Cobbe & Karon—parlayed an MVP season in Korea into a second shot in the Majors with a guaranteed three years and $15 million in Milwaukee.

Often players who find their Major League prospects stunted or even nonexistent will turn to jobs in Asia, where they can experience success and riches that go well beyond what the best Triple-A players can earn. Journeyman Dennis Sarfate became the most dominant closer in Japan and received a three-year extension worth more than $18 million, adding to the well over $10 million he already had made. Two of Sosnick, Cobbe & Karon’s other clients, Randy Messenger and Kris Johnson, each signed $10 million deals, a hefty haul in Nippon Professional Baseball. Kelly was well on his way to that.

Once he won a championship with SK Wyverns and their manager, former Royals skipper Trey Hillman, Kelly wanted to see what the major league market would bear. The answer was: plenty. The World Series champion Boston Red Sox were interested. So were the up-and-coming San Diego Padres. Kelly’s desire to return to his hometown—he was born and raised in the Phoenix area—and Arizona’s willingness to guarantee a second year sealed the homecoming.

Kelly could prove a solid bargain—perhaps not on the level of Miles Mikolas, who returned from Japan last year to dominate for the St. Louis Cardinals, but a back-of-the-rotation type that every team covets. And that’s fine with him. Every pitch Merrill Kelly throws in the big leagues, after all, he’s making history. (Jeff Passan—Yahoo Sports - 12/06/2018)

Merrill is an intelligent guy who speaks articulately. He is a cerebral pitcher. Kelly is confident, accomplished, and well prepared. 

Dec. 1, 2018: Merrill and Bre Carl were married.

In 2019, the Baseball America Prospect Handbook rated Kelly as the 15th-best prospect in the D'Backs organization.

MLB debut - March 30, 2019: It was a night of memorable debuts for the Arizona Diamondbacks.

Merrill Kelly paused for a moment before making his first big league appearance at age 30.

"I took a minute before I threw the first pitch and took in the stadium, took in the surroundings, just so I can have that memory before I stepped on the rubber," the right-hander said. "Once I got going, once I stepped on the rubber, it was game time."

Kelly threw six strong innings for the victory and benefited from an offensive outburst started by Adam Jones' leadoff homer to lead the Arizona Diamondbacks over the San Diego Padres 10-3.

2019 Season: After having only been a pitching prospect for the Tampa Bay Rays from 2010 to 2014, Merrill Kelly was a valuable starter for the Arizona Diamondbacks last year. He finished his age-30 season having been worth 2.0 WAR, with 183.1 innings pitched, 13 wins with a 4.42 ERA and 1.31 WHIP to go along with a 20.3% strikeout rate. The innings pitched really is what made Kelly so valuable, as well as his consistency, as 15 of his 32 starts went for quality starts (six-plus innings with three runs or less allowed).

Kelly ended up becoming a perfect comparison to Masahiro Tanaka. Both finished 2019 with 15 quality starts, Tanaka threw 1.2 innings less, had nine less strikeouts, and their WHIP and ERA were within one-tenth of each other. ( Jack Trent Dorfman - Fansided - Oct. 30, 2019)

The exception where he didn’t provide a quality start was his final outing of the year, going only five innings and allowing four runs in Oakland. He was scratched just before his next outing on August 24, with a “right shoulder nerve impingement”, which he said he had been dealing with for the previous ten days. But a couple of days later, it was announced Kelly had surgery to remove a blood clot in his shoulder, and to find the cause, he went to see specialist Dr. Gregory Pearl in Texas. This found Kelly to be suffering from vascular thoracic outlet syndrome, which is when “blood vessels or nerves [the former in Kelly’s case] in the space between your collarbone and your first rib (thoracic outlet) are compressed.”

The symptoms include shoulders and neck pain, as well as numbness in your fingers, and the procedure to relieve it involves taking a rib out to take pressure off the nerves. [No word on whether the removed rib will be used to create Mrs. Eve Kelly...] The process is much less potentially problematic than it used to be, and there are an increasing number of pitchers who have had the surgery and returned successfully, since Kenny Rogers was the first such, back in 2001. They include Matt Harvey, Clayton Richard and Tyson Ross. Chris Archer had it earlier this year. However, nothing medical is ever certain: in 2018, Jay Jaffe wrote on Fangraphs about the “sobering history” of pitchers post-TOS surgery.

By all accounts, the surgery went well, and Kelly was able to lean on both Archer and Alex Cobb, former team-mates when Merrill was in the Rays organization, who have been through the surgery. Cobb in particular proved a source of comfort, as he had suffered from the same variety of TOS, affecting the blood vessels and causing a blood clot. He told our pitcher he hasn’t had any issues since going through the process in 2011; Cobb started ten games for the Orioles this year. “That’s why talking to Alex really put my mind at ease,” Kelly said. If all goes well, Merrill will be able to begin throwing at the start of December, and should be ready to go come the start of spring training in February 2021.

The problem for the Diamondbacks is, they don’t get to wait until then to see how things are going, before making a decision on the option. That will happen shortly after the end of the World Series. Complicating matters further is that the team also holds a $5.25 million team option on Merrill’s services for 2022. According to Jack, “Each option [is] decided independently, do not need to be exercised at the same time.” So they don’t need to commit to two years of the pitcher immediately. But obviously, if this year’s one is declined, the team don’t then get any say at the end of 2021. Kelly would become an unrestricted free-agent, able to sign wherever he wants, for as long as he wants.

If Merrill were to return to health, and pitched at the level he had over 2019-20 combined - a 109 ERA+ - then this would be close to a no-brainer. League average starting pitching does not typically become available for $4.25 million, and he’d not need to be much better than replacement level to make it a worthwhile investment. After Zac Gallen, the D-backs rotation was perilously thin this year following the loss of Kelly: over those 35 games, Arizona starters were 5-15, with everyone else bar Gallen going 2-13. Merrill would seem to be a lock for a starter’s spot if healthy. But the key words there are, of course, “if healthy”. though GM Mike Hazen doesn’t appear to be too concerned, at least, publicly. (Jim McLennan@AZSnakepit  - Oct 14, 2020)

2015: Kelly signed with the SK Wyverns of the Korea Baseball Organization.

Kelly is a righthander with a 92-97 mph 2-seam SINKER that he throws for strikes. His hard, tight CURVEBALL is his best secondary pitch. He has an effective SLIDER. He also has a good CUTTER. (Spring, 2019)

2019 Season Pitch Usage: 4-seam Fastball: 36% of the time; Sinker 11.1%; Change 13.4%; Curve 21%; and Cutter 18.6% of the time. Average velocity: 4-seam 92.3 mph, Sinker 91.7, Change 86.3, Curve 80.9, and Cutter 90.2 mph.

Merrill had a lot of funk in his deceptive delivery when he was in the Rays organization. But he fixed his delivery, adding to his durability, becoming a starter in Korea.

On pitching for South Korea Wyvens, Kelly said, "It's helped me become a better pitcher, because over there sometimes you kinda have to figure out what you’re doing wrong. Or the learning curve on your own because of the language barrier or because off their mentality. Going over there has forced me to be my own pitching coach and I think that's why I was able to progress into who I am today.”

Kelly has 55 grade control and 50 command, on the 20-80 scouting scale.

According to The Athletic, Kelly's average fastball velocity increased in all four years in the KBO: 89.5 in 2015, 90.8in 2016, 91.8 in 2017, and 92.5 in 2018.

"One big thing is just the maturation of my body getting stronger as I got older,” Kelly said about his increased velocity. "The next part would be the progression of my mechanics. When I first came to Korea, I was not very good at using my whole body, including my legs, when delivering the ball. Over the past couple of years, I have tried to focus on using more of my body to throw."

June 12, 2019: Merrill Kelly showed once again why he might be ready to be counted on more heavily as the season progresses. Kelly tossed 7 2/3 innings as the D-backs beat the Phillies, 2-0 at Citizens Bank Park.

“He just did a really good job staying out of the middle of the plate,” Phillies catcher J.T. Realmuto said. “He was able to throw the cutter in on lefthanders and then away to righties, he kept it strike-to-ball all night. He did a good job of locating. He was able to bury his curveball when needed. He kept us off-balance and off the bases.”

Sept 20, 2019: Kelly has tweaked his mechanics just a bit, turning his body a little more when gathering himself during his windup, but the majority of his changes have come between his ears.

Kelly (12-14) allowed just two hits over seven shutout innings to extend his scoreless innings streak to 15. Kelly also threw seven shutout innings in his last start against the Reds and has allowed only three runs over his last 27 innings, spanning his four September starts.

“I think really it just came down to -- a lot of it was mentality,” Kelly said. “Just being aggressive and not giving the hitters too much credit and not trying to nitpick and throw perfect pitches. Just having the confidence that I’m good enough as myself and don’t need to be any better than what I’m doing is kind of what it boils down to.”

And instead of waiting to work his way into a game, Kelly has focused on being ready to go from the game’s first pitch.

“It finally took until the last month and a half of the baseball season to get to that point where he’s walking off the mound in the bullpen and standing on the [game] mound throwing his first pitch as his best pitch,” Lovullo said. “I think there’s a lot to be said for that. The velocity has climbed a little bit, and it’s maintained. I don’t think he’s working into his outings. He’s ready to go from the first pitch on.”

“It’s been a roller-coaster, for sure,” Kelly said. “There’s definitely been some highs and lows, definitely some learning experiences -- which I feel like your first year in the big leagues, that’s partly what it’s about is learning as much as you can, not only about the hitters but about yourself. I think that I’ve done that this year. I’m happy where we are now.” (S Gilbert - MLB.com - Sept 21, 2019)

Aug 24, 2020: Kelly was on the IL with right shoulder nerve impingement .

Aug 25, 2020: Merrill had a procedure to dissolve a blood clot in his pitching shoulder and is likely out for the remainder of the 2020 season, according to general manager Mike Hazen. Hazen said that the plan is for Kelly to take the next several days to a week to recover, and then head to Dallas to visit highly regarded vascular surgeon Gregory Pearl to be further evaluated.

“What caused the underlying issue needs to be answered,” Hazen said. “And that will happen with those next steps.”

The biggest question is whether there is a thoracic outlet issue like thoracic outlet syndrome, which, according to Johns Hopkins Medicine, is when nerves or blood vessels are compressed by the rib, collarbone or neck muscles at the top of the outlet.  (Gilbert - mlb.com)