Molloy grew up in the Australian Capital Territory (ACT) before moving to Melbourne in the mid-1980s to study performing arts at the University of Melbourne, where he wrote and performed in his first live act in 1987. He attended The Peninsula School during his secondary/high school period. It was during Molloy's time at university that he met the Australian comedy troupe, The D-Generation (the future cast of The Late Show), that consisted of members who would later form Working Dog Productions; Molloy would also collaborate with members on several projects.
Molloy worked with the D-Generation as a writer-performer on their 1988 Seven Network specials (he had auditioned for the D-Gen in 1986, but it wasn’t until the troupe saw him in the 1987 Melbourne University revue, Laminex on the Rocks, that they signed him up). He also worked as a writer on the ABC's The Gerry Connolly Show (1988), the Network Ten series, The Comedy Company, in its 1989 season, and, while working on the (unscreened) pilots for The Late Show for the Nine Network (1990), Molloy was a cast member of ABC TV’s The Late Show (1992–1993) as a writer/performer. As well as pairing up with Tony Martin for each episode’s introduction and the Street Interviews segment, Mick co-hosted the segments Muckrakers with Jason Stephens and Commercial Crimestoppers with Santo Cilauro, played the thick-witted assistant of stuntman Rob Sitch in Shitscared, and performed in countless sketches (he provided the voices of Sergeant Olden in The Olden Days and Chief Chromedome in Bargearse). Molloy also delivered a series of volatile rants in the Mick’s Serve part of Late Show News.
Molloy landed his own late-night TV show, The Mick Molloy Show, which premiered on the Nine Network in 1999. Planned to run for twenty episodes, it was slated by the critics and cancelled after only eight. Molloy's next involvement with television was the show, Any Given Sunday, with Nicole Livingstone, on the Nine Network in 2006. He was also involved in the 2006 Commonwealth Games coverage on the Nine Network, as a commentator for the lawn bowls events.
In 2007, Molloy filmed a pilot of The Nation, a weekly late-night news-based comedy hour on the Nine Network. The show debuted on 5 June, but struggled for ratings, attracting only 575,000 viewers nationally for its fourth show. Originally it was on Tuesdays at 9:30 pm, but was moved to Wednesdays at 10:30 pm and has been off air since the end of August 2007.
In 2008 it was announced that Molloy would replace Peter Helliar on the panel of Channel 10's Before the Game, an AFL themed show, after Helliar had discontinued his position post-2007. On the show, his views as a keen supporter of the Richmond Football Club (Tigers) were often made clear. Molloy is one of the club's more prominent supporters and as well as appearing on the Before the Game, he has in the past appeared in a club membership advertising campaigns. In June 2008, Molloy made comments on the show regarding Nicole Cornes, the wife of former Adelaide coach Graham Cornes, in which he suggested she had slept with footballer Stuart Dew. Nicole Cornes sued the Ten Network for defamation, and was awarded A$85,000 in July 2011. Molloy remained with Before the Game until it was axed at the end of the 2013 AFL season.
In 2009, Molloy starred in the sitcom The Jesters which aired on Movie Extra on Foxtel. He played a veteran comic Dave Davies who has become the manager of a group of young, rookie comedians and who has to cope with the behind-the-scenes antics of producing a comedy sketch show. A second series aired in 2011.
In February 2010 it was announced that Molloy and Eddie McGuire would be investigated by the NSW Anti-discrimination board following complaints that their on air coverage of the Vancouver Winter Olympics men's figure skating contained homophobic remarks.
In February 2014, it was announced by the Seven Network that Molloy would be joining its one-hour AFL match preview show on Saturday nights during the football season.. In 2015, Molloy was announced as a host of an online show called Friday Front Bar (now just The Front Bar). In 2016, Channel 7 picked up the show.
Molloy is a semi regular contestant on Channel 10's hit show Have You Been Paying Attention?. As of 7th July 2017, he has made 22 appearances on the show for only 1 win.
After writing and performing on The D-Generation Breakfast Show on the Triple M radio, from 1990 to 1992, Molloy teamed up with Tony Martin for Triple M’s Bulltwang (1990)
After The Late Show finished, Molloy developed, again with Martin, the successful Austereo radio programme, Martin/Molloy (1995–98), which produced three ARIA award winning compilation albums, The Brown Album (1995), Poop Chute (1996) and Eat Your Peas (1998). Molloy occasionally appeared with his former D-Generation cohorts on Network Ten’s The Panel (1998–2004).
Molloy hosted another radio show, Tough Love with Mick Molloy (2004–06) on Triple M alongside Robyn Butler and Richard Molloy.
In 2011, Molloy returned to radio and to Melbourne's Triple M, as a permanent member of The Hot Breakfast. He is also appearing daily at 6:50am on Triple M Brisbane's The Grill Team.
Following the cancellation of The Mick Molloy Show, Molloy returned with a video release, entitled Shonky Golf with Mick Molloy, and he directed the feature-length documentary Tackle Happy (2000). He played Kim's dad Gary Poole on Kath & Kim (2003–04) and co-starred, with David Wenham, in two Murray Whelan telemovies, Stiff and The Brush-Off (both 2004).
He has starred in three movies, Crackerjack (2002) (which he also co-wrote, receiving an AFI nomination), Tony Martin's Bad Eggs (2003), and BoyTown (2006), which, like Crackerjack, he co-wrote with his brother Richard Molloy. During production of the DVD-release for Boytown in 2007, Molloy and his long-time collaborator Tony Martin had a dispute over the proposed extra content for the DVD and the two have not worked together since. In the 2006 Australian feature film Macbeth, Molloy played Brown; it was the second time he has appeared in a production of Macbeth.
Molloy was the long-term partner of Australian actor Sophie Lee in the 1990s. (Lee regularly appeared on Molloy's nationally syndicated radio show, as well as playing Tracey Kerrigan in the film The Castle, which was made by Molloy's former D-Gen cohort).
He is a supporter of the Richmond Football Club.
In April 2012, at the 54th Logie Awards, Molloy delivered an onstage tribute to friend and colleague Bill Hunter, who died in 2011.
On The Hot Breakfast on 8 June 2012 Molloy announced he was to become a father for the first time, his partner being pregnant with twin boys. On 11 July he announced their birth. The boys were born on 10 July, and were a month early.The D-Generation Goes Commercial (1988) – Various – (also writer)The Gerry Connolly Show (1988) – (writer only)The Comedy Company (1989) – Various – (also writer)The Late Show (1992–1993) – Himself/Various – (also writer)The Mick Molloy Show (1999) – Himself – (also writer/producer)Kath & Kim (2003) – Gary Poole (2 eps)Stiff (2004) – Angelo AgnelliThe Brush-Off (1996) – Angelo AgnelliAny Given Sunday (2006) – HimselfThe Nation (2007) – Himself – (also writer/producer)Before The Game (2008–2013) – HimselfThe Jesters (2009, 2011) – Dave DaviesSaturday Night Football (pre-game show) (2014–present) – HimselfFriday Front Bar (post-game show) (2015–present) – HimselfTackle Happy (2000) – Himself – (also director/producer)Crackerjack (2002) – Jack Simpson – (also writer/producer)Bad Eggs (2003) – Ben KinnearMacbeth (2006) – BrownBoyTown (2006) – Tommy – (also writer/producer)BoyTown Confidential (2007) – Tommy Boy – (also writer/producer)The D-Generation Breakfast Show (Triple M Melbourne 1990–1992)Bulltwang (Triple M Melbourne 1990)Martin/Molloy (Austereo network 1995–1998)Tough Love with Mick Molloy (Triple M network 2004–2006)The Hot Breakfast (Triple M Melbourne 2011–present)The Grill Team (Triple M Sydney 2011–present)Martin/Molloy: The Brown Album (1995) 2CDMartin/Molloy: Poop Chute (1996) 2CDMartin/Molloy: Eat Your Peas (1998) 2CDShonky Golf (1999) – Himself – (also writer/director/producer)