Hamid became the first D.C. United Academy player to sign with the first team on September 2, 2009. Prior to signing with D.C. United, he was linked with a move to Celtic, until work permit issues derailed the move.

Hamid played for the United States at under-18 and under-20 level, and in October 2009 he trained with the senior team. Hamid made his international debut for U.S. national team on January 21, 2012, keeping a shutout as his team beat Venezuela 1–0 in an exhibition game at the University of Phoenix Stadium. He became the ninth goalkeeper not to concede on his international debut for the United States.

He made his professional debut on May 5, 2010 in a game against the Kansas City Wizards. With this win, Hamid became the youngest goalkeeper in MLS history, at 19 years 161 days, to win a regular season game, besting Tim Howard’s record by four days. Hamid started 28 of United’s 34 games during the 2011 season. He retained his starting spot during the 2012 season, making appearances in 27 matches.

After the 2011 regular season, Hamid began a 10-day trial on October 29, 2011 with English Premier League club West Bromwich Albion. He stayed with the club until November 7.

In 2012, Hamid earned a record 88 saves in arguably his best season in MLS to date. Despite Hamid being sent off with a red card during the 2012 playoffs in a match against the New York Red Bulls, D.C. United was able to come away with a win and advance to the next round. Hamid, emotional after seeing the win, was videotaped shirtless screaming “You can’t hold us back” while pounding his chest. The phrase became a rallying cry for D.C. United fans who organized a display featuring the quote. Although D.C. United failed to defeat their next challenger, the Houston Dynamo, the phrase stuck with the fans. For the 2013 season, the D.C. United fan group Screaming Eagles designed a scarf with the quote and presented Hamid with a free one. D.C. United won the 2013 Lamar Hunt U.S. Open Cup, the 100th edition of the competition. Hamid played the final, a 1–0 win at Real Salt Lake, having been deputized by Joe Willis in previous rounds. In 2014, D.C. United finished top of the Eastern Conference in the regular season, having finished bottom the year before. Hamid won the MLS Goalkeeper of the Year Award.

At the end of the 2017 season, Hamid’s contract with D.C. United expired as the two sides failed to negotiate a new contract. On October 25, 2017, it was announced that Hamid had signed with Danish Superliga club FC Midtjylland valid from January 1, 2018. He was back-up to Jesper Hansen in his first season, making only one league appearance on April 22 in a 3–3 draw at Aalborg, in which he made a mistake from a corner kick to give the opponents a 3–2 lead. The “Wolves” finished the campaign as league champions.

Hamid rejoined D.C. United on a year-and-a-half loan on August 8, 2018.

On December 9, 2019, it was announced that Hamid had return to D.C. United on a permanent basis for a reported transfer fee of $750,000.

On December 30, 2019, Hamid was called up to Gregg Berhalter’s 25-man roster for a friendly against Costa Rica on February 1, 2020.