Michael Ryan Pritchard was born in Berkely, California. He was given up for adoption by his biological mother, who struggled with heroin addiction. At six weeks of age, he was placed in foster care with Cheryl Nasser and Patrick Pritchard of El Sobrante, California, a couple who subsequently adopted him.
In the cafeteria in Carquinez Middle School, ten-year-old Dirnt met Billie Joe Armstrong, with whom he bonded instantly. Armstrong began to teach Dirnt how to play guitar, and the two spent much time in Armstrong's bedroom learning to play songs by Ozzy Osbourne and Van Halen. They formed a group with fellow students Raj Punjabi on drums and Jason Andrew Relva on bass, with Armstrong playing lead guitar and Dirnt playing rhythm guitar.
After attending Salesian High School, an all-boys Catholic school for his first year, Dirnt transferred to Pinole Valley High School, where Armstrong had also recently transferred from John Swett High School. Dirnt's family struggled with financial troubles; in an effort to help out, he worked as a chef at Nantucket, a seafood restaurant in Crockett. He eventually saved up enough money to purchase a used pickup truck that he and Armstrong often drove to Berkeley to attend shows at 924 Gilman Street, an influential DIY punk club. The pair got jobs at the club as security guards, despite their small physiques. Dirnt recalled, "We lived and died for that place. At that time, it meant everything.
While at Pinole Valley High School, Dirnt, Armstrong, and classmate Sean Hughes formed a band called Sweet Children. Dirnt's mother struggled to provide for him and his sister as a single parent, and eventually had to leave Rodeo in 1987 to look for work. Unwilling to leave Sweet Children, Armstrong and Dirnt convinced each other's parents to allow Dirnt to move into Armstrong's garage. After dummer John Kiffmeyer, who was also known as Al Sobrante, joined the group, Sweet Children began performing at 924 Gilman Street. The bang performed several well-received sets, which encouraged Armstrong to drop out of high school to focus his energy on music. Dirnt, however, worried that he needed a backup plan and continued his studies. Hughes, who was not as dedicated to the group as the other members, left Sweet Children, leaving Dirnt to play bass. Dirnt often brought his bass to school, and the plucking noise of the unamplified strings led classmates to jokingly call him by the onomatopoeia "Dirnt". After changing its name to Green Day, the band recorded its debut album 39/Smooth over the 1989 Christmas holiday break and went on its first van tour in June 1990, leaving the day that Dirnt graduated from high school.
Dirnt has a daughter, Estelle Desiree, who was born in April 1997 (with first wife Anastasia). He won full custody of her in the summer of 2008 and took her to live in Oakland. In 2004, he married his then-girlfriend Sarah. The two divorced that same year because Dirnt was spending too much time in the studio recording American Idiot. On March 14, 2009, he married Brittney Cade in a private ceremony in Brittney's hometown of Ojai, California. Dirnt has two children with Cade: a son, Brixton Michael (born October 11, 2008), and a daughter, Ruby Ryan Mae (born November 29, 2010).