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)

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)

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.”

“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)