He played in Major League Baseball (MLB) for the Washington Nationals, Houston Astros, and Boston Red Sox.

Peacock requested that his coach try him at pitcher, and he threw eight innings in high school prior to pitching in a summer league.

[2] He spent the 2007 season with the GCL Nationals, Washington's affiliate in the rookie-level Gulf Coast League, accruing a record of 1 win and 1 loss and a 3.89 earned run average.

After the 2010 season, Peacock played in the Arizona Fall League as a relief pitcher, and his fastball was measured at 97 miles per hour (156 km/h).

[6] On December 23, 2011, Peacock was traded with Tommy Milone, Derek Norris, and A. J. Cole to the Oakland Athletics for Gio González and Robert Gilliam.

After the 2012 season, the Athletics traded Peacock to the Houston Astros with Max Stassi and Chris Carter for Jed Lowrie and Fernando Rodriguez.

[9] He began the season as a relief pitcher, with a 1.10 ERA in 16+1⁄3 innings pitched, and received a spot start on May 22 in place of the injured Dallas Keuchel.

[11] The Astros won the American League West with 101 wins, and faced the Los Angeles Dodgers in the World Series.

In Game 3, Peacock earned his first major league save by completing the final 3+2⁄3 innings of a 5−3 win, allowing no hits and striking out four.

[17] The following day, the Red Sox selected Peacock's contract;[18] he started that evening's game against the Tampa Bay Rays,[19] taking the loss after allowing five runs on two hits in 2+1⁄3 innings.

[24] On March 12, 2024, Peacock was hired to serve as the pitching coach at his alma mater, Palm Beach Central High School.