Mullan had a surgery to remove bone spurs and loose bodies in his right ankle. Mullan scored his first goal of the 2010 season on May 1 in a 3–0 win over the Kansas City Wizards. On July 31, Mullan scored in the 90th minute to give a 2–2 draw with the New York Red Bulls. On September 5, in what ended up being his final game for the Dynamo, Mullan recorded an assist in a 2–1 loss to the San Jose Earthquakes. He had 3 goals and 3 assists in 22 league games in his last season in Houston. With the Dynamo struggling in the league, having the second fewest points in MLS at the time, combined with Mullan and his wife wanting to move back home to Colorado to be closer to family, Mullan approached Dynamo coach Dominic Kinnear about being traded to the Colorado Rapids. At the time of the trade, Mullan's 184 games played was the most in Dynamo history, although it now ranks 8th. Colorado Rapids On September 15, 2010, Mullan was traded along with a fourth-round pick in the 2013 MLS SuperDraft (later converted to a pick in the 2013 MLS Supplemental Draft) to the Colorado Rapids in exchange for Colin Clark and allocation money. He made his Rapids debut on September 18, getting the start in a 3–0 win over the New England Revolution. In the seven remaining regular season games after the trade, Mullan had 2 assists as he helped the Rapids pick up 3 wins and 3 draws, qualifying for the playoffs for the first time since 2006. Matched up with the Columbus Crew in the first round, Mullan had an assist in the first leg to help the Rapids earn a 1–0 win. The Rapids would lose the second leg 2–1, tying the aggregate score. With no away goals rule in place, the game went to penalty kicks, which Colorado won 5–4. The conference finals saw the Rapids come up against the San Jose Earthquakes, with Colorado winning 1–0 to advance to their first MLS Cup final since 1997. Mullan played all 120 minutes of the final, helping the Rapids beat FC Dallas 2–1 in extratime to win their first MLS Cup in franchise history. The victory made Mullan the second player in MLS history to win at least 5 MLS Cups, the first being his former Earthquake teammate Jeff