He played for the Los Angeles Galaxy, San Jose Earthquakes, Houston Dynamo, and Colorado Rapids in Major League Soccer.

[1][2] Mullan was born in Mineola, New York, but his family moved to Littleton, Colorado, a suburb of Denver, when he was young.

He also helped Creighton reach the NCAA Division I Men's Soccer Tournament final, their only appearance in the title game in school history.

On May 9, Mullan scored the first two goals of his pro career, with the second coming in the 92nd minute to give LA a 4–4 draw with the Tampa Bay Mutiny.

At MLS Cup 2001, Los Angeles fell to the San Jose Earthquakes 2–1, with Mullan appearing off the bench in the game.

On April 12, Mullan recorded an assist on Brian Ching's goal in the 1st minute to help the Earthquakes to a 2–1 win over the Colorado Rapids in San Jose's first game of the MLS season.

In their next game, he scored his first goal for the Quakes to give San Jose a 1–1 draw with the Kansas City Wizards.

On October 15, Mullan scored once and had one assist to give the Quakes a 3–1 win over the Los Angeles Galaxy in the final match of the regular season.

[22] He had 3 goals and 6 assists from 25 regular season appearances as he helped the Earthquakes win their first Supporters' Shield in club history.

On April 2, 2006, Mullan recorded two assists to help Houston beat the Colorado Rapids 5–2 in the inaugural game in Dynamo history.

Mullan had a goal and an assist in the conference final, helping Houston to a 3–1 win over Colorado and advance to MLS Cup 2006.

About 10 seconds after the ensuing kickoff, Mullan received the ball on the right wing and whipped in a cross that Brian Ching headed into the goal to level the score at one.

In the 2006 U.S. Open Cup, Mullan made two appearances and had one assist to help the Dynamo reach the semifinals, where they fell to the Los Angeles Galaxy 3–1.

Mullan ended the regular season with 1 goal and 3 assists from 28 appearances, helping the Dynamo to a 2nd-place finish in the Western Conference.

[3] Mullan played every minute of Houston's playoff run as they returned to MLS Cup, where they beat New England again, this time by a score of 2–1.

[24] After the season, Mullan was voted Dynamo Team Co-MVP along with goalkeeper Pat Onstad in honor of their good performances throughout the year.

[29] The Dynamo opened the 2008 season in the 2008 CONCACAF Champions' Cup, with Mullan appearing three times in the tournament as Houston reached the semifinal, where they lost 3–0 on aggregate to Deportivo Saprissa.

[30] In the opening game of the 2008 North American SuperLiga group, Mullan had a goal and an assist as Houston beat Atlante F.C.

[31] Mullan played in each game of the tournament and had one goal and two assists as he helped the Dynamo reach the final, where they lost to New England on penalties.

[33] On July 21, Mullan suffered an ankle injury during a 2–1 loss to Seattle Sounders FC in the U.S. Open Cup semifinal.

Mullan made 25 appearances and had 4 assists during the regular season, helping Houston finish second in the Western Conference and third overall, one point behind Supporters' Shield winners Columbus Crew.

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.

[37] 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.

In the seven remaining regular season games after the trade, Mullan had two assists as he helped the Rapids pick up three wins and three draws, qualifying for the playoffs for the first time since 2006.

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.

It was Mullan's first game in Seattle since he had injured Steve Zakuani in 2011 and Sounders fans booed him every time he touched the ball.

"[57] On August 4, Mullan played in his 300th MLS regular season game as the Rapids beat Real Salt Lake 1–0.

[64] Mullan did not appear during the final three months of the season, partly due to injuries, but also because Pareja preferred different players in that position.

[69] Mullan received his first cap for the United States on July 11, 2004, coming on as a substitute in the 72nd minute during a 1–1 draw with Poland in a friendly.

[71] Creighton Bluejays Los Angeles Galaxy San Jose Earthquakes Houston Dynamo Colorado Rapids Individual