The Baltimore Orioles drafted Johnson in the fifth round of the 2001 MLB draft. He made his major league debut in 2006, making one start towards the end of the season. He allowed 8 runs in 3 innings for the loss. In 2007, he again only made one Major League appearance. Johnson began the 2008 season in the Triple-ANorfolk Tidesstarting rotation, but was called up to the Orioles on April 12. Pitching middle and late relief, he ran up 18 consecutive scoreless innings before giving up a 10th inning run to Oakland on May 5. He became the interim closer when George Sherrill went on the disabled list on August 19, 2008. Johnson finished the season with a 2.23 ERA in 54 games with 1 save. Johnson was given the closer role on July 30, 2009, when Sherrill was traded to the Los Angeles Dodgers. In the 2010 season, he returned to a setup role. Late in the 2011 season, Johnson won the closer role from Kevin Gregg. He finished with 10 saves, but his ERA rose higher, finishing at 4.11 in 70 innings of relief. On January 16, 2012, Johnson signed a one-year deal worth $2.625 million, avoiding arbitration. He won the Delivery Man of the Month Award for May 2012. On July 1, 2012, Johnson was one of three Orioles selected to play in the 2012 All Star Game. On September 21, 2012, Johnson set the Orioles' all-time single season save record at 46, passing Randy Myers. Johnson recorded his 50th save of the season on September 30 during a victory over the Boston Red Sox. With the win, Baltimore clinched a post-season berth, and Johnson became the 10th pitcher in MLB history to record 50 saves or more. He finished the season with 51 saves. He won the AL Rolaids Relief Man Award. Johnson had a mediocre postseason debut through four games in the 2012 American League Division Series against the New York Yankees. Johnson struggled in game one of the series at Camden Yards, entering in the 9th inning of a 2–2 game and surrendering five runs in only of an inning. The Yankees won, 7–2. He gained redemption in game two, pitching a perfect 9th inning to preserve a 3–2 advantage, striking out Alex Rodriguez on a 3–2 count to end the game. Johnson pitched again in game three in the Bronx and surrendered a game-tying home run to Raúl Ibañez with one out in the 9th inning. Johnson earned a successful save in game four, working a scoreless 13th inning. Johnson recorded his 100th career save against the New York Yankees on June 30, 2013.
Oakland Athletics
On December 2, 2013, Johnson was traded to the Oakland Athletics for infielder Jemile Weeks and a player to be named later, identified as David Freitas on December 12. The deal was considered to be a salary dump by the Orioles, since Johnson was projected to make $10.8 million in baseball arbitration, according to MLB Trade Rumors. His 2014 salary wound up being 10 million dollars. Johnson's first season with the A's began poorly, as he earned a blown save and 2 losses in his first two appearances with the club. After 5 appearances and an ERA of 18.90, Johnson was pulled from the closer role on April 11, in favor of fellow relievers Luke Gregerson and Sean Doolittle. Johnson was designated for assignment on July 24, and released by the club on August 1. In 2014 for the A's, in 38 games he was 4-2 with 2 saves and a 7.14 ERA.
Detroit Tigers
On August 5, 2014, Johnson signed a minor-league contract with the Detroit Tigers. On August 16, the Tigers called up Johnson to the main roster. To make room on the 25-man roster, the Tigers optioned Melvin Mercedes back to Triple-A Toledo. To make room on the 40-man roster, the Tigers designated Kevin Whelan for assignment. Johnson made his debut for the Tigers on August 17, coming in to pitch the sixth inning. Johnson allowed two hits, three runs, one walk, and one strikeout in innings. In 2014 for the Tigers, in 16 games he was 1-0 with a 6.92 ERA.
Atlanta Braves
Johnson signed a one–year contract worth $1.6 million with the Atlanta Braves on December 3, 2014. Johnson, who had been the set-up man for most of the year, was called on to be the closer after Jason Grilli was injured on July 11, two days before the All-Star break. In 49 games, he was 2–3 with a 2.25 ERA and nine saves.
Los Angeles Dodgers
On July 30, 2015, in a three-team trade, the Los Angeles Dodgers acquired Johnson, Mat Latos, Michael Morse, Bronson Arroyo, Alex Wood, Luis Avilán, and José Peraza, while the Miami Marlins acquired minor league pitchers Victor Araujo, Jeff Brigham, and Kevin Guzman, and the Braves received Héctor Olivera, Paco Rodriguez, minor league pitcher Zachary Bird and a competitive balance draft pick for the 2016 MLB draft. He struggled with the Dodgers, allowing 22 runs in 18 innings. He did not make the postseason roster and the Dodgers designated him for assignment on October 14, 2015.
Second stint with the Braves
On November 30, 2015, Johnson signed a one-year, $2.5 million deal to return to the Braves. For the final week of July 2016, Johnson earned NL Player of the Week honors. He finished the 2016 season with a 3.06 ERA, his lowest since 2013. He also saved 20 games. He then signed a two-year extension on October 2, 2016. He struggled the following season with the Braves, posting a 5.56 ERA despite saving 22 games for the Braves but also blew 9 saves.
Los Angeles Angels
On November 30, 2017, the Braves traded Johnson and $1.21 million in international bonus signing cash to the Los Angeles Angels in return for minor league pitcher Justin Kelly. In his first season with the Angels, Johnson appeared in 62 games, registering an ERA of 3.84 in innings. He elected free agency on October 29.
Pitching style
Johnson throws four pitches, leading with a hard sinker averaging. His main off-speed pitch to right-handed hitters is a curveball averaging about. Against lefties, Johnson adds an upper-80s changeup. Lastly, he throws an occasional four-seam fastball in the mid 90s.
