[10]: 8 [11] Biographer Patrick McGilligan stated in his book Jack's Life that Latvian-born Eddie King (originally Edgar A. Kirschfeld),[12] June's manager, may have been Nicholson's biological father, rather than Furcillo.

[10] In 1957, Nicholson joined the California Air National Guard,[16] a move he sometimes characterized as an effort to "dodge the draft";[17] the Korean War era's Military Selective Service Act was still in force, and draftees were required to perform up to two years of active duty.

[19] He trained to be an actor with a group called the Players Ring Theater, after which he found small parts performing on the stage and in TV soap operas.

Corman directed Nicholson on several occasions, such as in The Little Shop of Horrors as undertaker (and masochistic dental patient) Wilbur Force; in The Raven; The Terror, where he plays a French officer seduced by an evil ghost; and The St. Valentine's Day Massacre.

Nicholson frequently worked with director Monte Hellman on low-budget westerns; two of them—Ride in the Whirlwind and The Shooting—initially failed to interest U.S. film distributors but gained cult success on the French art-house circuit and were later sold to television.

His first real taste of writing success was the screenplay for the 1967 counterculture film The Trip (directed by Corman), starring Peter Fonda and Dennis Hopper.

[21] Biographer John Parker writes that Nicholson's interpretation of his role placed him in the company of earlier antihero actors, such as James Cagney and Humphrey Bogart, while promoting him into an "overnight number-one hero of the counter-culture movement".

"[10]: 130  He told his new agent, Sandy Bresler, to find him unusual roles so he could stretch his acting skill: "I like to play people that haven't existed yet, a 'cusp character'", he said, "I have that creative yearning.

"[31] In 1974, Nicholson starred in Roman Polanski's noir thriller Chinatown, and was again nominated for the Academy Award for Best Actor for his role as Jake Gittes, a private detective.

At the time, Nicholson was out of town making a film, but his steady girlfriend, actress Anjelica Huston, had dropped by unannounced to pick up some items.

[38] The role seemed perfect for Nicholson, with biographer Ken Burke noting that his "smartass demeanor balances his genuine concern for the treatment of his fellow patients with his independent spirit too free to exist in a repressive social structure".

[17] : 273  Reviewer Marie Brenner notes that his bravura performance "transcends the screen" and continually inspires the other actors by lightening their mental illnesses with his comic dialogue.

She describes his performance: Nicholson is everywhere; his energy propels the ward of loonies and makes of them an ensemble, a chorus of people caught in a bummer with nowhere else to go, but still fighting for some frail sense of themselves.

Nicholson plays a journalist, David Locke, who during an assignment in North Africa decides to quit journalism and disappear by taking on a new hidden identity.

[42] Critic and screenwriter Penelope Gilliatt provides an overview of Nicholson's role, "The Passenger is an unidealized portrait of a drained man whose one remaining stimulus is to push his luck.

[45] Nicholson has observed that while both De Niro and Brando were noted for their skill as method actors, he himself has seldom been described as one, a fact he sees as an accomplishment: "I'm still fooling them", he told Sean Penn.

Although he garnered no Academy Award for Stanley Kubrick's adaptation of Stephen King's The Shining (1980), his role in the film as writer Jack Torrance remains one of his more significant.

[17] : 318 Nicholson won his second Oscar, an Academy Award for Best Supporting Actor, for his role of retired astronaut Garrett Breedlove in Terms of Endearment (1983), directed by James L. Brooks.

[27] For his role as hot-headed Col. Nathan R. Jessup in A Few Good Men (1992), a movie about a murder in a U.S. Marine Corps unit, Nicholson received yet another Academy Award nomination for Best Supporting Actor.

"[60] In 1996, Nicholson collaborated once more with Batman director Tim Burton on Mars Attacks!, pulling double duty as two contrasting characters, President James Dale and Las Vegas property developer Art Land.

[61][62] David Thomson states that the film was terribly neglected, since Nicholson portrayed one of his best screen characters, someone who is "snarly, dumb, smart, noble, rascally—all the parts of 'Jack'".

"[65] His Oscar was matched by the Academy Award for Best Actress for Helen Hunt, who played a Manhattan single mother drawn into a love/hate friendship with Udall, a frequent diner in the restaurant where she works as a waitress.

[28] The win was Nicholson's third Academy Award, tying him with six other actors, Walter Brennan, Ingrid Bergman, Meryl Streep, Daniel Day-Lewis, and Frances McDormand who all have three acting Oscars.

[67] That same year Nicholson starred in The Pledge, a mystery drama where he portrays retired police detective Jerry Black, who vows to find a murderer of a young girl.

[68] Nicholson acted in Alexander Payne's comedy-drama About Schmidt (2002), playing a retired Omaha, Nebraska, actuary who questions his own life after his wife dies.

In 2003, Nicholson also starred in the Nancy Meyers directed romantic comedy Something's Gotta Give playing 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 in Martin Scorsese's Oscar-winning film The Departed, a remake of Andrew Lau's Infernal Affairs, playing Frank Costello, a nefarious Boston Irish Mob boss based on Whitey Bulger, who was still on the run at the time.

[109] After Thompson died in 2005, Nicholson and fellow actors Johnny Depp, John Cusack, and Sean Penn attended his private memorial service in Colorado.

[115] After the death of former Lakers star Kobe Bryant in a helicopter crash in January 2020, Nicholson gave a rare phone interview to Los Angeles station KCBS-TV expressing his grief.

Only Nicholson (1960s–2000s), Michael Caine (1960s–2000s), Meryl Streep (1970s–2010s), Paul Newman (1950s–1960s, 1980s–2000s), Katharine Hepburn (1930s–1960s, 1980s), Frances McDormand (1980s–2020s), Denzel Washington (1980s–2020s), and Laurence Olivier (1930s–1970s) have been nominated for an acting (lead or supporting) Academy Award in five different decades.