[10] In November 2012, Curtin was named assistant coach for his hometown Philadelphia Union, under John Hackworth and alongside Brendan Burke and Rob Vartughian.

[11] During the 2014 season, Hackworth was fired after a run of poor results with Curtin being named interim coach during the team's search for a permanent replacement.

[12] The Union saw a turnaround of form under Curtin, losing only 5 of the remaining 16 matches in the season and advanced to the U.S. Open Cup final, ultimately finishing runners-up to Seattle Sounders FC in extra time.

This success was rewarded on November 7, 2014, the Philadelphia Union announced that Curtin would take the reins as the head coach; removing the "interim" title he had held previously.

Near the end of the 2015 season, the Union fired Nick Sakiewicz as CEO and appoint Earnie Stewart as "Sporting Director" to work with Curtin and address the team's struggles.

This success lead to the club announcing Curtin signed a two-year contract extension, to remain head coach through the 2021 season.

Despite disruptions from the COVID-19 pandemic, the Union reached the semi-final of the MLS is Back Tournament and went on to won their first trophy finishing top of the league standings and earning the 2020 Supporters' Shield.