He played in Major League Baseball (MLB) for the New York Yankees, Miami Marlins, Seattle Mariners, Toronto Blue Jays, Chicago Cubs, Milwaukee Brewers, and Philadelphia Phillies.

[1] Though Phelps was ranked as the sixth best prospect from Missouri prior to the 2005 Major League Baseball draft, Phelps was not selected due to his strong commitment to enroll at the University of Notre Dame, where he pitched for the Notre Dame Fighting Irish baseball team.

Based on Oppenheimer's recommendation, the Yankees selected Phelps in the 14th round of the 2008 Major League Baseball draft.

On December 19, 2014, the Yankees traded Phelps and Martín Prado to the Miami Marlins for Nathan Eovaldi, Garrett Jones, and Domingo Germán.

[13] At the start of the 2016 season, Phelps was installed as the Marlins primary setup reliever, usually called in to pitch the eighth inning.

Through 25 games, he maintained an ERA of 1.93[14] On July 20, 2017, the Marlins traded Phelps to the Seattle Mariners for prospects Brayan Hernandez, Brandon Miller, Pablo Lopez, and Lukas Schiraldi.

The Blue Jays traded Phelps to the Chicago Cubs on July 30, 2019, in exchange for minor league player Thomas Hatch.

On August 31, 2020, the Brewers traded Phelps to the Philadelphia Phillies in exchange for Juan Geraldo, Brandon Ramey, and Israel Puello.

[23] On May 25, it was announced that Phelps would miss the remainder of the season after undergoing surgery to repair a “significant” strain in his right lat.

On November 29, Phelps signed a minor league contract with the Blue Jays and was invited to spring training.

[27] Phelps is a devout Catholic and frequently attended Mass at St. Patrick's Cathedral in Midtown Manhattan when he was a member of the Yankees.