Krol was drafted by the Oakland Athletics in the seventh round of the 2009 Major League Baseball Draft out of Neuqua Valley High School in Naperville, Illinois. He was selected despite not pitching in his senior year due to being suspended for the entire season for an alcohol-related offense, his second violation of the district athletic code of conduct. After missing time due to an elbow injury, Krol made his professional debut on August 27, starting for the AZL Athletics. After that, he made three appearances with the Low-A Vancouver Canadians. Krol played most of 2010 with Single-A Kane County Cougars, where in 24 games, he went 9–4 with a 2.65 ERA, striking out 91 in 118 innings, and was a mid and post-season all-star. He also made four starts for High-A Stockton at the end of the year, going 1–0. Krol missed the first three months with an elbow injury, and was pitching in a rehab assignment in the Arizona League before he was suspended for the rest of the season—more than two months—for a tweet that included a graphic "gay slur along with some other offensive language". Krol began 2012 with the Ports as a starter, where in 15 starts, he went 1–7 with a 5.62 ERA. In late July, he was moved to the bullpen to reduce his projected innings pitched total. He performed well in that role, earning a promotion to the Double-A Midland RockHounds. In 29 combined appearances, he went 2–9 with a 5.20 ERA, striking out 89 batters in 97 innings.
Washington Nationals
Krol was acquired by the Washington Nationals in March 2013 as a player to be named later in the trade that sent Michael Morse to the Seattle Mariners, John Jaso to Oakland, and A. J. Cole and Blake Treinen to the Nationals. He was assigned to Double-A Harrisburg to begin the year, where he got off to a hot start. In 21 games with the Senators, he had a 0.69 ERA, striking out 29 in 26 innings. He was called up to the majors for the first time on June 4, 2013. He made his major league debut the next day against the Mets, striking out the side in one scoreless inning of work. After a short stint with the Triple-A Syracuse Chiefs at the end of August, Krol returned to the Nationals for September. In 32 games with Washington, he went 2–1 with two holds and a 3.95 ERA, striking out 22 in 27 innings.
Detroit Tigers
On December 2, 2013, Krol was traded to the Detroit Tigers, along with infielder Steve Lombardozzi and pitcher Robbie Ray, for starting pitcher Doug Fister. On July 31, 2014, Krol was optioned to the Tigers AAA affiliate, the Toledo Mud Hens. He was called up by the Tigers on September 8, 2015 as a September call-up. In 33 games with Detroit, Krol went 2–3 with a 5.79 ERA and 26 strikeouts in 28 innings.
Atlanta Braves
On November 20, 2015, the Tigers traded Krol and pitcher Gabe Speier to the Atlanta Braves in exchange for outfielder Cameron Maybin and cash considerations. He was invited to spring training, and sent down to begin the 2016 season with the Triple A Gwinnett Braves.
Los Angeles Angels
On February 6, 2018, Krol signed a minor league deal with the Los Angeles Angels. After being designated for assignment on May 28, he requested and received his release on May 31.
On June 18, 2019, Krol signed a minor league deal with the Minnesota Twins. He became a free agent following the 2019 season.
Pitch selection
Krol throws four pitches. He has a four-seam fastball in the 93–95 MPH range, a cut fastball between 88–90 MPH, a curveball between 77–79 MPH, and an above-average changeup averaging 83–85 MPH.
