In addition to helping with the family business, all of the Hammer children played baseball at the encouragement of their father, who had been a shortstop for Lewis–Clark State College in Idaho.

He made his first start for the new team on February 27, striking out six batters and allowing six hits in seven innings, while Michael Taylor picked up the win in Marshall's 2–1 victory over the Towson Tigers.

[13] Hammer's next win came on May 9, when he set personal highs with 11 strikeouts in eight innings, allowing six hits and three walks as the Herd defeated the Western Kentucky Hilltoppers 5–3.

[14] Hammer finished the 2015 season with a 2–6 record, 3.83 ERA, and one save in 15 appearances, 12 of which were starts, and he struck out 56 batters in a team-leading 80 innings.

[15] Hammer picked up his first win of the 2016 college baseball season on March 6, when he struck out nine batters and allowed three earned runs in six innings of a 12–6 victory over Lehigh.

[17] Hammer missed nine games with an upper body injury but finished the season 4–2 with a 4.88 ERA in 14 appearances, 11 of which were starts, and with 56 strikeouts in 66+1⁄3 innings.

After going 3–1 with a 1.08 ERA, 40 strikeouts, and six saves in his first 25 innings, he became one of six Tourists named to the South Atlantic League Mid-Season All-Star Team that June.

[26] On July 26, 2017, the Philadelphia Phillies traded sidearm reliever Pat Neshek and cash to the Rockies in exchange for three prospects: Hammer, Alejandro Requena, and Jose Gomez.

[27] Hammer was assigned to the High–A Clearwater Threshers of the Florida State League (FSL), becoming a bullpen staple with a 2–0 record and a 0.57 ERA in 12 appearances.

Impressed with his regular season performance, the Phillies sent Hammer to the Arizona Fall League to continue working on his pitch mechanics as a member of the Glendale Desert Dogs.

[33] He made his MLB debut the following day, pitching a scoreless inning against the Milwaukee Brewers by getting out Christian Yelich, Ryan Braun, and Mike Moustakas.

[35] Upon his return, Hammer's fastball velocity sharply declined with the rigors of a full season of professional baseball, and in his last 16 outings, he recorded a 14.49 ERA, .345 batting average against, and opposing batters had a 1.128 on-base plus slugging against him.

[40] Hammer opened the 2021 season with the IronPigs, posting a 1.74 ERA and striking out 33 batters in 20+2⁄3 innings before he was called up to Philadelphia on July 10.

He made his season debut that day against the Boston Red Sox, loading the bases on a hit by pitch, a single and a walk before retiring the next three batters to escape the inning without allowing a run.

[49] He made his Frontier League debut in the Crushers' season opener, pitching a scoreless inning in relief against the Québec Capitales on May 12.

[50] The Crushers also deployed Hammer as a starting pitcher, and he picked up his first win on June 21, pitching five innings in a shutout of the Schaumburg Boomers.

[53] He made his debut with the Triple-A Salt Lake Bees on August 3 as a starter, allowing one run in five innings against the El Paso Chihuahuas.

[33] He began wearing them during the 2017 season after his poor vision prevented him from reading the signs indicating which pitches his catcher was calling for him to throw.

Garrett Hammer played baseball for Rocky Mountain High School, Kellogg Community College, and New Mexico Highlands University.