As a child, his father was often away obtaining a degree, while his mother stayed at home to care for him and his sister Myla. Dirnt excelled in school despite missing classes, often as a result of various illnesses believed to be caused by his biological mother's drug use. After an argument between his parents resulted in a call to the police, the couple divorced. His mother and sister moved to Rodeo, CA while he stayed in El Sobrante, CA with his father. However, missing his mother, he eventually moved in with her and Myla. Having been previously described as bright and fearless, Dirnt became sullen and withdrawn after the divorce.
In the cafeteria in Carquinez Middle School, ten-year-old Dirnt met Billie Joe Armstrong, with whom he bonded instantly. Armstrong began to play guitar with Mike, and the two spent much time in Armstrong's house and they played songs by various artists, most notably The Ramones and Van Halen. At the age of 15, they formed a group named "Sweet Children" with Billie Joe singing and playing lead guitar and Mike 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 and classmate Billie Joe Armstrong 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 drummer John Kiffmeyer, who was also known as Al Sobrante, joined the group, Sweet Children began performing at 924 Gilman Street. The band 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. Bass player Sean 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.
While performing at the 1998 KROQ Weenie Roast in Irvine, California, Third Eye Blind bassist Arion Salazar ran onstage and "bear-hugged" Dirnt, who was caught off-guard. The incident escalated into an on-stage scuffle before security took Salazar away. After the performance, Dirnt confronted Salazar backstage, and as the two argued, a beer bottle struck Dirnt in the head, causing a small fracture in his skull. Eyewitnesses later attributed the bottle throwing to a fan of Third Eye Blind. Salazar and the band's management soon released a statement: "I am sorry that my attempt at doing something I thought would be funny escalated into Mike getting hurt. That was never my intention. I simply had too much to drink and made a very bad decision. If I had been in Mike's place, I am sure I would have acted similarly. My heart goes out to him and I hope he recovers quickly."
In the live album, Bullet in a Bible, Armstrong calls Dirnt "The best bass player in the history of punk rock."
In 2012, during the iHeartRadio Music Festival, Dirnt and Armstrong smashed their respective bass and guitar after Armstrong became agitated onstage and ranted about Green Day's set being cut short for Usher.
Although Armstrong is Green Day's main lyricist, Dirnt has written lyrics for "Emenius Sleepus," "J.A.R. (Jason Andrew Relva)", "Scumbag", "Ha Ha You're Dead," the subtrack "Nobody Likes You" from the medley "Homecoming" and the b-side "Governator," He also co-wrote the lyrics to "Best Thing in Town," "The One I Want," and "Panic Song" with Armstrong, and writes all the bass lines for Green Day songs. Mike also composed the intro to "Desensitized" from Shenanigans using a baseball bat and an old cathode ray tube television.
Dirnt sings lead vocals on "Governator," part of the third verse of "Outsider" on the Shenanigans album, the "Nobody Likes You" subtrack of "Homecoming." He also sings the subtrack "Modern World" from the medley "American Eulogy" from 21st Century Breakdown and sings lead for exactly 10 seconds in "Sex, Drugs, and Violence" from ¡Tré!.
Dirnt married his first wife, Anastasia Serman in 1996, until their divorce in 1999. Anastasia and Dirnt had a daughter together, Estelle Desiree, who was born on December 20, 1996. He won full custody of her in the summer of 2008 and took her to live in Oakland. In 2004, he married his girlfriend Sarah Garrity in a private villa in Puerto Vallarta, Mexico. The two divorced that same year. 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, Ryan Ruby Mae (born November 29, 2010).
Dirnt is a Star Wars fan; he stated jokingly in an interview that he bases "most of his religious beliefs" on Star Wars. Dirnt's favorite bass player was Cliff Burton. He is a part owner of Rudy's Can't Fail Cafe, a diner in Emeryville and Oakland. The diner was named after a song by punk rock band The Clash, "Rudie Can't Fail", from their album London Calling. The menu item names are loosely based on punk themes such as "God Save the Chicken", a reference to "God Save the Queen" by the Sex Pistols, and "Give 'Em Enough Meatloaf", a reference to Give 'Em Enough Rope by The Clash.
Dirnt was adopted; his biological mother died on January 9, 2013. Dirnt only managed to meet her one-month before her death.
In July 2014, it was announced that Dirnt would co-produce the Indie film Crickets, with Green Day manager, Pat Magnarella.
In December 2015, Dirnt along with Billie Joe Armstrong, announced that they would be launching a coffee company, Oakland Coffee Works. The company will sell organic coffee beans and is said to be the first company to use exclusively 100% compostable bags and pods.
Dirnt supported Bernie Sanders during the 2016 Presidential Campaign.39/Smooth (1990) – bass guitar, backing vocals
Kerplunk! (1991) – bass guitar, backing vocals
Dookie (1994) – bass guitar, backing vocals
Insomniac (1995) – bass guitar, backing vocals
Nimrod (1997) – bass guitar, backing vocals, baseball bat (as written in the album's liner notes)
Warning (2000) – bass guitar, backing vocals, farfisa on "Misery"
American Idiot (2004) – bass guitar, backing vocals, lead vocals on "Nobody Likes You" and the non-album track "Governator"
21st Century Breakdown (2009) – bass guitar, backing vocals, lead vocals on "Modern World"
¡Uno! ¡Dos! ¡Tré! Trilogy (2012) – bass guitar, backing vocals and lead vocals on part of "Sex, Drugs and Violence"
Revolution Radio (2016) - bass guitar, backing vocals
Stop Drop and Roll!!! (2008) – bass guitar, vocals
Bored in the USA (EP) (2000) – bass guitar, backing vocals
Achtung Jackass (2002) – bass guitar, backing vocals
Griller (EP) (2011) – bass guitar, backing vocals
How to Make Enemies and Irritate People (1994) – bass, backing vocals
"Suzanne Is Getting Married" (single) (1994) – bass and backing vocals on track 1
Squirtgun (1995) – bass and backing vocals on "Make It Up"
King of the Hill – (TV series – 1997) – teenager
Riding in Vans with Boys (film – 2003) – himself
Live Freaky! Die Freaky! (film – 2006)
The Simpsons Movie (film – 2007) – himself
Heart Like a Hand Grenade (film – 2008) – himself
Green Day: Rock Band (video game – 2010) – himself (Voice, likeness, and archive footage)
¡Cuatro! (film – 2013) as himself
Broadway Idiot (film – 2013) as himself
Anchorman: The Legend Continues (film – 2013) – himself