He had to add the "Junior" to the end of his name to distinguish himself from his father, Larry Mullen, Sr. As Larry's career blossomed, so did his tax bills and his father was the unlucky recipient of them in the early 1980s. Before U2, Larry's previous gigs included playing drums for the Post Office Workers Union Band, and, for three weeks, the Artane Boys Band. Some of his earliest gigs included the St. Patrick's Day Parade on O'Connell Street, the bandstand on St. Stephen's Green, and the pier in Dun Laoghaire.

Larry's music career started when he began taking piano lessons when he was 8 years old. He did not fancy the notion of studying the piano scales or learning music theory, so he gave up the piano and began drumming in 1971. He started taking classes with Ireland's best-known drummer, Joe Bonnie. When Bonnie died a year later, his daughter Monica continued teaching Larry. However, Larry has said that his drumming style is "unteachable" and that spirit and instinct are what guides his technique. He has said that he just wanted to "physically hit the thing," in regard to the drums, so lessons where he couldn't just play were not his cup of tea, so to speak. Larry did return to the piano when he played keyboards on "Yahweh" during the Vertigo tour.

His sister, Cecilia, bought him his first drum-kit in 1973 for £17. He placed a notice at the infamous Mount Temple Comprehensive School in the fall of 1976, and on September 25, 1976, the band auditions began in his kitchen in Artane. Although everyone knows the band as U2, Larry claims that the band's name is really "The Larry Mullen Band."

Growing up, Larry considered his life to be "pretty normal for a while." However, Larry's oldest sister, Mary, died in 1973. Five years later, his mother, Maureen, died in a road traffic accident in November 1978. He says in U2 by U2, "In some ways, both events defined the kind of person I've become. My mother's death certainly catapulted me in the band's direction."

Larry left school in 1978 after passing his Intermediate Certificate exams. Larry said he was offered an opportunity to complete his Leaving Certificate exams, but chose not to as the economy was not doing very well at the time and jobs were difficult to find. While the band was still trying to score a record deal in 1978, Larry worked at Seiscom Delta in the purchasing department for a year. Had he stayed at Seiscom, his career path would have been computer programming for Seiscom's geology department.

In the early days of U2, Larry had to miss some gigs and photo shoots because of the job at Seiscom. For the photo sessions he could not make, friend-of-the-band Derek "Guggi" Rowen stepped in as he resembled Larry to a degree. For the gigs, Larry arranged a stand-in named Eugene from a north Dublin rock band called Stryder. When Larry became injured on the job, running over his toe with his motorbike, Eric Briggs filled in for him on the drums. Bono says in U2 by U2 that there was a period of time where the other three band members almost kicked Larry out of the band because they were not sure how serious he was. Ironically, during the band's first recording session, a CBS record executive suggested that Larry needed to be fired because of his inability to keep tempo.

Also during the early days, Bono spoke to Larry about a local Christian-based fellowship group. Prior to that, Larry grew up with a traditional Catholic background, even serving as an altar boy in church. Larry, Bono and Edge attended the Bible studies with some of their friends from Lypton Village and became friendly with members of the Shalom Christian group. When Larry, Bono, and Edge were told that they should give up rock-and-roll a few years later, they chose to leave the Christian group, instead. Larry said about Shalom in U2 by U2, "The idea was to create a Christian community, where people would live and work under strict Christian standards, When you're young and impressionable it all sounds ideal. But there was something terribly wrong with the concept. It was a bit like the bigger the commitment you made, the closer you were to heaven. It was a really screwed-up view of the world and nothing to do with what I now understand a Christian faith to be. There was huge pressure to follow that path and what made it even stranger was that rather than it coming from the church leaders, it was coming from our friends. I learned a lot though and I also gained a faith I didn't have before, and that's still with me."

He has been with his partner, Ann Acheson, for almost 30 years. They met during Larry's first year at Mount Temple. Together they have two sons and one daughter: Aaron Elvis (born October 4, 1995), Ava (born December 23, 1998) and Ezra (born in February 2001). He has also been a dog owner, thanking his Labradors (JJ and Missy) on past albums. Larry rarely speaks about his personal life, and has been relatively successful in keeping his family life private.