Strickland was called up to the majors for the first time on September 1, 2014. He pitched one scoreless inning of relief against the Colorado Rockieslater that day. In the 2014 regular season, Strickland appeared in 9 games, allowing 5 hits and no runs in 7.0 innings pitched. In Game 1 of the 2014 National League Division Series versus the Washington Nationals, Strickland entered the game with the bases loaded and two outs in the sixth inning and struck out Ian Desmond to preserve the Giants' lead. In Game 2, Strickland recorded the save in the longest playoff game in Major League history, as the Giants defeated the Nationals 2–1 in 18 innings. However, Strickland set a postseason record for a reliever by allowing six home runs, even though the Giants went on to defeat the Kansas City Royals in the 2014 World Series.
2015
Strickland started the 2015 season with the Triple-A Sacramento River Cats, posting a 1.66 ERA in 21.2 innings, before being called up on May 24, 2015. Strickland did not yield a home run until August 3, a string of 188 batters between Triple-A and the majors. In 2015, Strickland appeared in 55 games, recording a 2.45 ERA with 50 strikeouts in 51.1 innings pitched. Strickland's 0.857 WHIP was the third-lowest among relievers with at least 50 innings pitched.
2016
In 2016, Strickland was named to the opening day Major League roster for the first time in his career. He remained in the Giants bullpen throughout the season, collecting an ERA in the low 3's.
2017
In a May 29, 2017, game against the Washington Nationals, Strickland hit Bryce Harper's right hip with a 98-mph fastball, which resulted in a benches-clearing brawl. Strickland had to be physically removed from the field by teammates after exchanging several punches with Harper. He received widespread criticism for his role in starting the brawl, as sports analysts and Harper have suggested that the intentional hit-by-pitch was the result of a grudge from the two home runs by Harper against him in the 2014 National League Division Series, a series the Giants ended up winning in 4 games en route to a World Series championship. The next day, on May 30, he was suspended for six games.
2018
With Mark Melancon heading to the disabled list at the beginning of the season, Strickland was named the Giants closer. He earned his first save in a 1-0 victory on Opening Day against the Los Angeles Dodgers. While playing against the Miami Marlins on June 18, Strickland allowed three runs, blowing a save, and the Giants lost 5–4. Shortly after a pitching change, Strickland punched a door out of frustration, which wound up fracturing his right hand. Strickland underwent surgery the very next day, where the pinkie finger on his right hand was repaired. Strickland was ruled out for 6-8 weeks.
Seattle Mariners
On January 24, 2019, Strickland signed a one-year deal with the Seattle Mariners.. On March 30, Strickland suffered a Grade 2 right lat strain, which put him out for a couple of months.
Washington Nationals
On July 31, 2019, the Mariners traded Strickland and Roenis Elías to the Washington Nationals in exchange for Aaron Fletcher, Taylor Guilbeau, and Elvis Alvarado. In 24 games with the Nationals, Strickland went 2-0 with a 5.14 ERA. Overall in 2019, combined with both teams, Strickland made 28 total relief appearances with a 2-1 record and a 5.55 ERA. The Nationals finished the 2019 year with a 93-69 record, clinching a wild card spot, and eventually went on to win the 2019 World Series over the Houston Astros, their first championship in franchise history. It was also the second time Strickland won a World Series in his career. During the NLDS against the Los Angeles Dodgers, he gave up his 9th career postseason home run, a record for relief pitchers. He was released by the Nationals on March 14, 2020.
New York Mets
On June 29, 2020, Strickland signed a minor league contract with the New York Mets organization. On July 23, it was announced that Strickland had made the Opening Day roster for the Mets. On July 30, Strickland was designated for assignment when Brian Dozier was added to the roster.
Personal life
Strickland married Shelley Todd in November 2011. The couple had their first child, a daughter, in April 2017.
