Mullen played one season for the Sioux Falls Stampede in the United States Hockey League (USHL) before attending the University of Denver (DU).

Mullen played three seasons for the Kings minor league affiliate the Manchester Monarchs, before leaving as a free agent and signing with the Vancouver Canucks.

[10][11] In his senior season, Mullen set career highs in assists and points while leading Denver in blocked shots.

[5][12] Undrafted out of college, Mullen signed a two-year entry-level free agent contract with the Los Angeles Kings on 3 April 2009.

[13] Playing with the Kings farm team the Manchester Monarchs, Mullen recorded his first professional point, an assist, in his first game, a 6–3 victory over the Albany River Rats.

[16] He finished the year playing 8 more games for the Monarchs bring his total to 44 and recorded 4 goals and 10 points in his first professional season.

[21] With the 2012–13 lockout approaching Mullen was assigned to the Canuck's American Hockey League affiliate, the Chicago Wolves.

[25] In the 2015–16 season, his third within the Senators' organization, Mullen was traded by Ottawa to the Nashville Predators in exchange for Conor Allen on 14 January 2016.

[28] On 2 February 2017, having agreed to return to the AHL with the Binghamton Senators, Mullen was claimed off waivers by the Rochester Americans and played out the remainder of the 2016–17 season in collecting 14 points in 27 games.

[29] As a free agent, Mullen opted to return abroad in agreeing to a two-year contract with Swedish outfit, Linköpings HC of the SHL on 10 July 2017.

[30] On 15 February 2018, he left Sweden for German DEL side Adler Mannheim, joining the team on loan for the remainder of the 2017–18 campaign.