Their mother, Audree, forced Brian to include Dennis in the original lineup of the Beach Boys. Urged by older cousin Mike Love, Dennis had approached Brian to form a group and compose a song about surfing. The Beach Boys formed in August 1961, with Murry taking over as manager, and were immediately successful. Though the Beach Boys developed their image based on the California surfing culture, Dennis was the only actual surfer in the band. For the sleeve notes of their 1964 album All Summer Long, Dennis wrote: “They say I live a fast life. Maybe I just like a fast life. I wouldn’t give it up for anything in the world. It won’t last forever, either. But the memories will.”

In the early years of the Beach Boys, Brian gave him the role of the drummer. Dennis quickly learned the basics of drumming at school lessons and, like the other members, he picked up more on the job. Brian took note of Dennis’s limited drumming technique early on and, as the mid-1960s approached, often hired session drummers, such as Hal Blaine, to perform on studio recordings (additionally substituting all other players at one time or another, under the demand for the band members on tour). Dennis accepted this situation with equanimity, generally giving high praise to his older brother’s work, as Brian’s compositions became more mature and complex. Early in 1963, Dennis teamed with Brian’s former collaborator Gary Usher, a neighbor in Hawthorne who became a prolific creative figure in surf music recording and, later, folk. As a duo writing, producing, and performing, and calling themselves the Four-Speeds, they released the single “RPM” backed with “My Stingray”.

In 1967, Dennis was cited as “the closest to brother Brian’s own musical ideals … He always emphasises the fusion, in their work, of pop and classical music.” Dennis said his brother Brian was an “inspiration”, not an influence, and that “Musically, I’m far apart from Brian. He’s a hundred times more than what I am musically.”

Wilson was initially fascinated by Manson and his followers, referring to him as “the Wizard” in a Rave magazine article at the time. The two struck a friendship, and over the next few months, members of the Manson Family – mostly women who were treated as servants – were housed at Wilson’s household, costing him approximately $100,000 (equivalent to $740,000 in 2019). Much of those expenses went into cars, clothes, food, and penicillin shots for their persistent gonorrhoea. In late 1968, he told the magazine Record Mirror that “when I met [Charlie] I found he had great musical ideas. We’re writing together now. He’s dumb, in some ways, but I accept his approach and have [learned] from him.” Some of Manson’s songs were recorded at Brian’s home studio. Those recordings remain unheard by the public. Wilson also introduced Manson to a few friends in the music business, including the Byrds’ producer Terry Melcher.

In September 1968, Wilson recorded a Manson song for the Beach Boys, originally titled “Cease to Exist” but reworked as “Never Learn Not to Love”, as a single B-side released the following December. It was credited solely to Wilson. When asked why Manson was not credited, Wilson explained: “He didn’t want that. He wanted money instead. I gave him about a hundred thousand dollars’ worth of stuff.” According to Beach Boys collaborator Van Dyke Parks: “One day, Charles Manson brought a bullet out and showed it to Dennis, who asked, ‘What’s this?’ And Manson replied, ‘It’s a bullet. Every time you look at it, I want you to think how nice it is your kids are still safe.’ Well, Dennis grabbed Manson by the head and threw him to the ground and began pummeling him … I heard about it, but I wasn’t there.” Writing in his 2016 memoir, Mike Love recalled Wilson saying he had witnessed Manson shooting a black man “in half” with an M16 rifle and hiding the body inside a well.

Wilson distanced himself from Manson and moved out of the house, leaving Manson there. When Manson subsequently sought further contact, he left a bullet with Wilson’s housekeeper to be delivered with a threatening message. In August 1969, Manson Family members perpetrated the Tate–LaBianca murders. Shortly afterward, Manson visited Wilson’s home, telling him that he had “just been to the moon” and demanded money, which Wilson gave. That November, Manson was apprehended and later convicted for numerous counts of murder and conspiracy to commit murder. Wilson refused to testify against Manson. He explained: “I couldn’t. I was so scared.” Instead, he was privately interviewed by prosecutor Vincent Bugliosi. Wilson’s testimony was deemed inessential since Jakobson agreed to publicly testify and corroborate Wilson’s claims.

Dennis continued writing songs for the Beach Boys’ subsequent albums, including Sunflower (1970), which featured the single “Forever” and three other songs written by Dennis. Their inclusion was said to be at the insistence of the label, which claimed that his songs sounded more contemporary than other rejected Beach Boys tracks. On December 4, 1970, Dennis released his first piece of solo material, an obscure single released only in Europe and the UK under the credit “Dennis Wilson & Rumbo”. The single featured “Sound of Free”, on his usual theme of freedom, on the A-side with the romantic “Lady” (also known as “Fallin’ In Love”) on the B-side. The song was later covered by American Spring and released as the B-side to their single “Shyin’ Away”.

In 1971, Dennis starred alongside James Taylor and Warren Oates in the film Two-Lane Blacktop as “The Mechanic”. The film depicts “The Driver” (Taylor) and “The Mechanic” driving aimlessly across the United States in their 1955 Chevy, surviving on money earned from street racing. That same year, he injured his hand badly enough to prevent him from playing drums for some time, so Ricky Fataar took over as the group’s drummer between 1972 and 1974. During this period Dennis acted as a co-frontman alongside Mike Love, as well as playing keyboards and singing. The live album The Beach Boys in Concert (1973) features only Dennis onstage among thousands of fans on the album cover.

During the three-year recording hiatus following Holland, Dennis’s voice deteriorated markedly. By then his onstage antics (including streaking) occasionally disrupted the Beach Boys’ live shows. In 1974, concurrent with the success of the ’60s hits compilation Endless Summer, Dennis returned to his role behind the drums. According to Dennis’s biographer, Jon Stebbins, it was this year that he co-wrote the lyrics and modified part of the melody of “You Are So Beautiful” at a party with Billy Preston.

In the subsequent years, Wilson allegedly received death threats from members of the Manson Family, who telephoned his home and told him: “You’re next”. In 1976, he commented that “I don’t talk about Manson. I think he’s a sick fuck. I think of Roman and all those wonderful people who had a beautiful family and they fucking had their tits cut off. I want to benefit from that?” In the 1978 biography The Beach Boys and the California Myth, Wilson acknowledged the interest in his relationship with Manson and said: “I know why [he] did what he did. Someday, I’ll tell the world. I’ll write a book and explain why he did it.” According to biographer Mark Dillon: “Some attribute [Wilson’s] subsequent spiral of self-destructive behavior ― particularly his drug intake ― to these fears and feelings of guilt for ever having introduced this evil Wizard into the Hollywood scene.”

Wilson’s first wife was Carole Freedman, with whom he had a daughter Jennifer and adopted son Scott from her previous relationship. His second was Barbara Charren, with whom he had two sons, Michael and Carl. Dennis was then married twice to actress Karen Lamm, the ex-wife of Chicago keyboardist Robert Lamm, in 1976 and again in 1978. He also had a relationship with Christine McVie of Fleetwood Mac.

By 1977, Dennis had amassed a stockpile of songs he had written and recorded while factions within the Beach Boys became too stressful for him. He expressed: “If these people want to take this beautiful, happy, spiritual music we’ve made and all the things we stand for and throw it out the window just because of money, then there’s something wrong with the whole thing and I don’t want any part of it.” He then approached James William Guercio, owner of Caribou Records, who stipulated “a structured recording process” before signing Dennis to a two-album contract. According to Guercio: “My discussions with Dennis were along the lines of, ‘You just tell Gregg [Jakobson] what you need – you have the studio and your job is to finish the dream. Finish the vision. Trish Roach [personal assistant] will do the paperwork and Gregg’s the co-ordinator. It’s your project… You’ve got to do what Brian used to do. Use anybody you want – it’s your decision and you’re responsible.”

Dennis released his debut solo album Pacific Ocean Blue in 1977. It sold poorly, peaking at No. 96 on the US Billboard album chart. Dates were booked for a Dennis Wilson solo tour, but these were ultimately cancelled when his record company withdrew concert support in light of poor album sales and a perception that he was becoming increasingly unreliable. He did occasionally perform his solo material on the 1977 Beach Boys tour. Despite Dennis claiming the album had “no substance”, Pacific Ocean Blue received positive reviews and later developed status as a cult item.

In succeeding years Dennis abused alcohol and heroin. Following a confrontation on an airport tarmac, he declared to Rolling Stone on September 3, 1977, that he had left the Beach Boys: “They kept telling me I had my solo album now, like I should go off in a corner and leave the Beach Boys to them. The album really bothers them. They don’t like to admit it’s doing so well; they never even acknowledge it in interviews.” Two weeks later, disputes were resolved, and Dennis rejoined the group. At some time, Brian’s then-girlfriend and nurse Carolyn Williams accused Dennis of enticing Brian to purchase about $15,000 worth of cocaine. When Brian’s bodyguard Rocky Pamplin and the Wilsons’ cousin Stan Love learned of this incident, they physically assaulted Dennis at his home. For the assault, they were fined about $1,000, and Dennis filed a restraining order.

Pacific Ocean Blue’s follow-up, Bambu, began production in 1978 at Brother Studios in Santa Monica, with the collaboration of then Beach Boys keyboardist and Dennis’s close friend Carli Muñoz as songwriter and producer. The first four songs officially recorded for Bambu were Muñoz’s compositions: “It’s Not Too Late”, “Constant Companion”, “All Alone”, and “Under the Moonlight”. The project was initially scuttled by lack of financing and the distractions of simultaneous Beach Boys projects. Bambu was officially released in 2008 along with the Pacific Ocean Blue reissue. This material was also released on vinyl in 2017, without Pacific Ocean Blue, for Record Store Day.

As the Beach Boys pressured Brian to readmit himself into Eugene Landy’s Twenty-Four Hour Therapy program, Dennis was informed by friends that he would be the band’s next target, to Dennis’s disbelief. He was proven wrong as the rest of the band gave him an ultimatum after his last performance in November 1983 to check into rehab for his alcohol problems or be banned from performing live with them.

By November 1983, Dennis was homeless and living a nomadic life. He checked into a therapy center in Arizona for two days, and then on December 23, checked into St. John’s Medical Hospital in Santa Monica, where he stayed until the evening of December 25. Following a violent altercation at the Santa Monica Bay Inn, Dennis checked into a different hospital in order to treat his wounds. Several hours later, he discharged himself and reportedly resumed drinking immediately. On December 28, three weeks after his 39th birthday, Dennis drowned at Marina Del Rey after drinking all day and then diving in the afternoon to recover his ex-wife’s belongings, previously thrown overboard at the marina from his yacht three years earlier amidst their divorce. Forensic pathologist Michael Hunter believed that Dennis experienced shallow-water blackout just before his death.

On January 4, 1984, the U.S. Coast Guard buried Dennis’s body at sea, off the California coast. The Beach Boys released a statement shortly thereafter: “We know Dennis would have wanted to continue in the tradition of the Beach Boys. His spirit will remain in our music.” His song “Farewell My Friend” was played at the funeral.

In 1988, Dennis was inducted into the Rock and Roll Hall of Fame posthumously as a member of the Beach Boys.

Dennis’s widow Shawn Love reported that Dennis had wanted a burial at sea, and his brothers Carl and Brian did not want Dennis cremated. At the time, only veterans of the Coast Guard and Navy were allowed to be buried in US waters without being first cremated, but Dennis’s burial was made possible by the intervention of then-President Ronald Reagan. In 2002, Brian expressed unhappiness with the arrangement, believing that Dennis should have been given a traditional burial.

The album remained largely out of print between the 1990s and 2000s. In June 2008, it was reissued on CD as an expanded edition. It was voted the 2008 “Reissue of the Year” in both Rolling Stone, and Mojo magazines and made No. 16 on the British LP charts and No. 8 on both the Billboard Catalog chart and the Billboard Internet Sales chart.