Nicholson was born in Manhattan, New York City, the son of a showgirl, June Frances Nicholson (stage name June Nilson). June had married Italian American showman Donald Furcillo (stage name Donald Rose) six months earlier in Elkton, Maryland, on October 16, 1936. Elkton was a town known for its "quickie" marriages. Furcillo however, was already married, and, although he offered to take care of the child, June's mother Ethel insisted that she bring up the baby, partly so that June could pursue her dancing career. Although Donald Furcillo claimed to be Nicholson's father and to have committed bigamy by marrying June, biographer Patrick McGilligan, who wrote Jack's Life (published in December 1995) asserted that Latvian-born Eddie King (originally Edgar A. Kirschfeld), June's manager, may be the father and other sources have suggested that June Nicholson was unsure of who the father was. Nicholson has chosen not to have a DNA test or to pursue the matter. Nicholson's mother was of Irish and Dutch descent though he and his family self-identified as Irish. Nicholson was brought up believing that his grandparents, John Joseph Nicholson (a department store window dresser in Asbury Park, New Jersey) and Ethel May Rhoads (a hairdresser, beautician and amateur artist in Neptune, New Jersey), were his parents. Nicholson only discovered that his "parents" were actually his grandparents and his sister was in fact his mother in 1974, after being informed by a Time Magazine journalist who was doing a feature on him.[9]By this time, both his mother and grandmother had died (in 1963 and 1970, respectively). Nicholson has stated he does not know who his father is, saying "Only Ethel and June knew and they never told anybody." Nicholson was raised in his mother's Catholic religion. Nick, as he was known to his high school friends, attended nearby Manasquan High School where he was voted "class clown" by the Class of 1954. A theatre and a drama award at the school are named in his honor. In 2004, Nicholson attended his 50 year high school reunion accompanied by his aunt Lorraine.

When Nicholson first came to Hollywood, he worked as a go-fer for animation legends Hanna-Barbera. Seeing his talent as an artist, they offered Nicholson a starting level position as an animation artist. However, citing his desire to become an actor, he declined. Nicholson started his career as an actor, writer, and producer, working for and with Roger Corman, among others. This included his screen debut in The Cry Baby Killer (1958), where he played a juvenile delinquent who panics after shooting two other teenagers, The Little Shop of Horrors (1960), in which he had a small role as a masochistic dental patient, and roles in two other Roger Corman films The Raven (1963) and The Terror (his first directing role for one day) (1963), co-starring then-wife Sandra Knight. As the 60s progressed, and with acting jobs still not easy to find, Nicholson began writing more often. The result of this included Thunder Island (1963), Flight to Fury (1964), Ride in the Whirlwind (1965), and The Monkees' vehicle Head (1968, co-written with director Bob Rafelson). These films enjoyed little if any success, but the young Nicholson was finally working more steadily. In the TV sitcom world, he also made appearances in two episodes of The Andy Griffith Show as Marvin Jenkins in 1966-1967.

With his acting career heading nowhere, Nicholson seemed resigned to a career behind the camera as a writer/director. His first real taste of writing success was the LSD-fueled screenplay for 1967's The Trip, which starred Peter Fonda and Dennis Hopper. However, after a spot opened up in Fonda and Hopper's Easy Rider, it led to his first big acting break. Nicholson played hard-drinking lawyer George Hanson, for which he received his first Oscar nomination. The part of Hanson was a lucky break for Nicholson -- the role had in fact been written for actor Rip Torn, who was a close friend of screen writer Terry Southern, but Torn withdrew from the project after a bitter argument with the film's director Dennis Hopper, during which the two men almost came to blows. A Best Actor nomination came the following year for his persona-defining role in Five Easy Pieces (1970), which includes his famous chicken salad dialogue about getting what you want. Also that year, he appeared in the movie adaptation of On A Clear Day You Can See Forever, although most of his performance was left on the cutting room floor. Other early, notable Nicholson roles included Hal Ashby's The Last Detail (1973), for which he was awarded Best Actor at the Cannes Film Festival, and the classic Roman Polanski noir thriller, Chinatown (1974) (he was Oscar-nominated for Best Actor in a Leading Role for both films). He also starred in The Who's Tommy (1975), directed by Ken Russell, and Michelangelo Antonioni's The Passenger (1975).

Nicholson earned his first Best Actor Oscar for portraying Randle P. McMurphy in the movie adaptation of Ken Kesey's novel One Flew Over the Cuckoo's Nest, directed by Miloš Forman in 1975. His Oscar was matched when Louise Fletcher received the Best Actress Award for her portrayal of Nurse Ratched. After this, he began to take more unusual roles. He took a small role in The Last Tycoon, opposite Robert De Niro. He took a less sympathetic role in Arthur Penn's western The Missouri Breaks, specifically to work with Marlon Brando. He followed this by making his second directorial effort with the western comedy Goin' South. His first movie as a director was a 1971 quirky release called Drive, He Said. Although he did not garner any Academy Award attention for Stanley Kubrick's adaptation of Stephen King's The Shining (1980), it remains one of Nicholson's most significant roles. His next Oscar, the Academy Award for Best Supporting Actor, came for his role of retired astronaut Garrett Breedlove in Terms of Endearment (1983), directed by James L. Brooks. Nicholson continued to work prolifically in the 80s, starring in such films as The Postman Always Rings Twice (1981), Reds (1981), Prizzi's Honor (1985), The Witches of Eastwick (1987), and Ironweed (1987). Three Oscar nominations also followed (Reds, Prizzi's Honor, and Ironweed). Nicholson turned down the role of John Book in Witness. The 1989 Batman movie, wherein Nicholson played The Joker, was an international smash hit, and a lucrative percentage deal earned Nicholson about $60 million. Nicholson was to reprise his role as The Joker in the fifth installment in the franchise Batman Triumphant in 1999, but Warner Bros. Pictures canceled the project. For his role as hot-headed Col. Nathan R. Jessep in A Few Good Men (1992), a movie about a murder in a U.S. Marine Corps unit, Nicholson received yet another Academy nomination. This film contained the court scene in which Nicholson famously explodes, "You can't handle the truth!", in one of the Aaron Sorkin-penned soliloquies to become part of popular culture. Not all of Nicholson's performances have been well received. He was nominated for Razzie Awards as worst actor for Man Trouble (1992) and Hoffa (1992). However, Nicholson's performance in Hoffa also earned a Golden Globe nomination. Nicholson would go on to win his next Academy Award for Best Actor for his role as Melvin Udall, a neurotic author with Obsessive Compulsive Disorder (OCD), in the romance As Good as It Gets (1997), again directed by James L. Brooks. Nicholson's Oscar was matched with the Academy Award for Best Actress for Helen Hunt as a Manhattan waitress drawn into a love/hate friendship with Udall, a frequent diner in the restaurant in which she worked. In 2001, Nicholson was the first actor to receive the Stanislavsky Award at the Moscow International Film Festival for "conquering the heights of acting and faithfulness".

In About Schmidt (2002), Nicholson portrayed a retired Omaha, Nebraska actuary who questions his own life and the death of his wife shortly afterward. His quiet, restrained performance stood in sharp contrast to many of his previous roles, and earned him an Academy Award Nomination for Best Actor. In the comedy Anger Management, he plays an aggressive therapist assigned to help overly pacifist Adam Sandler. In 2003, Nicholson starred in Something's Gotta Give, as an aging playboy who falls for the mother (Diane Keaton) of his young girlfriend. In late 2006, Nicholson marked his return to the "dark side" as Frank Costello, a sadistic Boston Irish Mob boss presiding over Matt Damon and Leonardo DiCaprio in Martin Scorsese's Oscar-winning The Departed, a remake of Andrew Lau's Infernal Affairs. In November 2006, Nicholson began filming his next project, Rob Reiner's The Bucket List, a role for which he shaved his head. The film starred him and Morgan Freeman as dying men who fulfill their list of goals. The film was released on December 25, 2007 (limited) and January 11, 2008 (wide). In researching the role, Nicholson visited a Los Angeles hospital to see how cancer patients coped with their illnesses. As of March 2008, it has been rumoured that Paul Thomas Anderson has been keen to write and direct Robert Evans' Power Play with Nicholson in the lead role.

In his adult personal life, Nicholson has been notorious for his inability to "settle down", with a place on Maxim's "Top 10 Living Legends of Sex." He has allegedly had sex with 2,000 women. He has five children by four different women; he was married once. Nicholson married Sandra Knight on June 17, 1962. The couple had one daughter, Jennifer Nicholson (born 1963) before divorcing on August 8, 1968. His other daughter, Honey Hollman (born 1981), was with Danish model Winnie Hollman. He has one son, Caleb Goddard (born 1970), with actress Susan Anspach, his Five Easy Pieces co-star. He had two children from his relationship with Rebecca Broussard: Lorraine Nicholson (born 1990) and Raymond Nicholson (born 1992). He has been romantically linked to numerous actresses and models, including Michelle Phillips, Bebe Buell, and Lara Flynn Boyle. Nicholson's longest relationship was for 17 years to actress Anjelica Huston, from 1973 to 1989, the daughter of film director John Huston. However, the relationship ended when the news reported that Rebecca Broussard had become pregnant with his child. He is also a close friend of film director Roman Polanski, whom he has supported through many personal crises including the death of his wife, Sharon Tate, at the hands of the Manson Family. He also supported Polanski through his conviction for statutory rape, a crime which took place on the Nicholson estate on Mulholland Drive. Nicholson lived next door to Marlon Brando for a number of years on Mulholland Drive in Beverly Hills. Warren Beatty also lived nearby, earning the road the nickname "Bad Boy Drive". After Brando's death in 2004, Nicholson purchased his neighbor's bungalow for exactly $5 million, with the purpose of having it demolished. Nicholson stated that it was done out of respect to Brando's legacy, as it had become too expensive to renovate the "derelict" building which is plagued by mold. During a road rage incident in 1994, he allegedly smashed another driver's car window with a golf club.

Nicholson is a fan of big-market teams like the New York Yankees and Los Angeles Lakers. His attendance at Lakers games is legendary, as he has courtside season tickets for the past 25 years at both The Forum and the Staples Center. In a few instances, Nicholson has engaged in arguments with game officials and opposing players, and has even walked onto the court. His ardent refusal to miss a Lakers home game means that studios must schedule filming around the Lakers home schedule. On February 4, 2008, he announced his endorsement of Senator Hillary Clinton in her race for the President of the United States. In an interview on Rick Dees' radio program, Nicholson said, "Mrs. Clinton has been involved in issues, everything from health care, which we know and prison reform and helping the military, speaking for women and speaking for Americans." California Governor Arnold Schwarzenegger and First Lady Maria Shriver announced on May 28, 2008 that Nicholson will be inducted into the California Hall of Fame, located at The California Museum for History, Women and the Arts. The induction ceremony will take place December 10th and he will be inducted alongside 11 other legendary Californians.

Nicholson has been nominated for an acting (lead or supporting) Academy Award in five different decades: 1960s, 1970s, 1980s, 1990s, and 2000s. The only other actors who can say the same are Michael Caine and Paul Newman. With 12 nominations thus far (8 for Best Actor and 4 for Best Supporting Actor), Jack Nicholson is the most nominated male actor in Academy Awards history. With three Oscar wins, he also ties with Walter Brennan for the 2nd highest number of Oscar wins in acting categories (all of Brennan's wins were for Best Supporting Actor). At the 79th Academy Awards, Nicholson had fully shaved his hair for his role in The Bucket List. Those ceremonies represented the seventh time he has presented the Academy Award for Best Picture (1972, 1977, 1978, 1990, 1993, 2006, and 2007). Every time he has won an Oscar, the lead actress of that same film has also won an Oscar. After he won an award for One Flew Over the Cuckoo's Nest at the award show he said," Man that Nurse Ratched is a bitch!"