Hartman was born Philip Edward Hartmann (later dropping one "n") on September 24, 1948, in Brantford, Ontario, Canada. He was the fourth of eight children of Doris Marguerite (Wardell) and Rupert Loebig Hartmann, a salesman specializing in building materials. His parents were Catholic and raised their children in that faith. As a child Hartman found affection hard to earn and stated: "I suppose I didn't get what I wanted out of my family life, so I started seeking love and attention elsewhere."

He and his family moved to the United States in 1958, gaining American citizenship in 1990. The family first lived in Connecticut, and later moved to the West Coast. There, Hartman attended Westchester High School and frequently acted as the class clown.

After graduating, Hartman studied art at Santa Monica City College, dropping out in 1969 to become a roadie with a rock band. He returned to school in 1972, this time studying graphic arts at California State University, Northridge. He developed his own graphic arts Business, which he operated on his own, creating over 40 album covers for bands including Poco and America, as well as advertising and the logo for Crosby, Stills & Nash. In the late 1970s, Hartman made his first television appearance on an episode of The Dating Game; he won but was stood up by his date.

Hartman married Gretchen Lewis in 1970, and they divorced sometime before 1972. He married real estate agent Lisa Strain in 1982, and their marriage lasted three years. Strain told People that Hartman was reclusive off screen and "would disappear emotionally [...] he'd be in his own world. That passivity made you crazy." Hartman married former model and aspiring Actress Brynn Omdahl (born Vicki Jo Omdahl) in November 1987, having met her on a blind date the previous year. Together they had two children, Sean and Birgen Hartman. The marriage had difficulties — Brynn reportedly felt intimidated by her husband's success and was frustrated she could not find any on her own, although neither party wanted a divorce. Hartman considered retiring to save the marriage. He tried to get Brynn acting roles but she became progressively more reliant on narcotics and alcohol, entering rehab several times. Because of his close friendship with SNL associate Jan Hooks, Brynn joked on occasion that Hooks and Hartman were married "on some other level".

Working alone as a graphic Artist, Hartman frequently amused himself with "flights of voice fantasies". Citing the need for a more social outlet for his talents, Hartman, aged 27, began in 1975 to attend evening comedy classes run by the California-based improvisational comedy group The Groundlings. While watching one of the troupe's performances, Hartman impulsively decided to climb on stage and join the cast. After several years of training, paying his way by re-designing the group's logo and merchandise, Hartman formally joined the cast of The Groundlings; by 1979 he had become one of the show's stars.

Hartman met Comedian Paul Reubens and the two became friends, often collaborating on writing and comedic material. Together they created the character Pee-wee Herman and developed The Pee-wee Herman Show, a stage performance which also aired on HBO in 1981. Hartman played Captain Carl on The Pee-wee Herman Show and returned in the role for the children's show Pee-wee's Playhouse. Reubens and Hartman made cameos in the 1980 film Cheech & Chong's Next Movie. Hartman co-wrote the script of the 1985 feature film Pee-wee's Big Adventure and had a cameo role as a reporter in the film. Although he had considered quitting acting at the age of 36 due to limited opportunities, the success of Pee-wee's Big Adventure brought new possibilities and changed his mind. After a creative falling-out with Reubens, Hartman left the Pee-Wee Herman project to pursue other roles.

Hartman wrote a number of screenplays that were never produced. In 1986 he began writing a screenplay for a film titled Mr. Fix-It, and completed the final draft in 1991. Robert Zemeckis was signed to produce the film, with Gil Bettman hired to direct. Hartman called it "a sort of a merger of horror and comedy, like Beetlejuice and Throw Momma From the Train", adding, "It's an American nightmare about a family torn asunder. They live next to a toxic dump site, their water supply is poisoned, the mother and son go insane and try to murder each other, the father's face is torn off in a terrible disfiguring accident in the first act. It's heavy stuff, but it's got a good message and a positive, upbeat ending." Zemeckis could not secure studio backing, however, and the project collapsed. Another movie idea involving Hartman's Groundlings character Chick Hazard, Private Eye also fell through.

Backstage at SNL, Hartman was called "the Glue", a name coined by Adam Sandler, according to Jay Mohr's book Gasping for Airtime. However, according to a biography on Hartman's life entitled You Might Remember Me: The Life and Times of Phil Hartman written by Mike Thomas, author and staff Writer for the Chicago Sun-Times, the nickname was created by SNL cast member and Hartman's frequent on-screen collaborator Jan Hooks. Hartman often helped other cast members. For Example, he aided Jan Hooks in overcoming her stage fright. SNL creator Lorne Michaels explained the reason for the name: "He kind of held the show together. He gave to everybody and demanded very little. He was very low-maintenance." Michaels also added that Hartman was "the least appreciated" cast member by commentators outside the show, and praised his ability "to do five or six parts in a show where you're playing support or you're doing remarkable character work". Hartman won the Primetime Emmy Award for Outstanding Writing for a Variety, Music or Comedy Program for SNL in 1989, sharing the award with the show's other Writers. He was nominated in the same category in 1987, and individually in 1994 for Outstanding Individual Performance in a Variety or Music Program.

Hartman first performed his Clinton impression on an episode of The Tonight Show. When he met Clinton in 1993 Hartman remarked, "I guess I owe you a few apologies", adding later that he "sometimes [felt] a twinge of guilt about [his Clinton impression]". Clinton showed good humor and sent Hartman a signed photo with the text: "You're not the President, but you play one on TV. And you're OK, mostly." For his Clinton impression, Hartman copied the president's "post-nasal drip" and the "slight scratchiness" in his voice, as well as his open, "less intimidating" hand gestures. Hartman opted against wearing a larger prosthetic nose when portraying Clinton, as he felt it would be distracting. He instead wore a wig, dyed his eyebrows brighter and used makeup to highlight his nose. One of Hartman's more famous sketches as Clinton saw the President visit a McDonald's restaurant and explain his policies by eating other customers' food. The Writers told him that he was not eating enough during rehearsals for the Sketch – by the end of the live performance, Hartman had eaten so much he could barely speak.

After his co-stars Jon Lovitz, Dennis Miller, Jan Hooks and Dana Carvey had left, Hartman said he felt "like an athlete who's watched all his World Series teammates get traded off into other directions [...] It was hard to watch them leave because I sort of felt we were all part of the team that saved the show." This cast turnover contributed to his leaving the show in 1994. Hartman had originally planned to leave the show in 1991, but Michaels convinced him to stay to raise his profile; his portrayal of Clinton contributed to this goal. Jay Leno offered him the role of his sidekick on The Tonight Show but Hartman opted to stay on SNL. NBC persuaded him to stay on SNL by promising him his own comedy–variety show entitled The Phil Show. He planned to "reinvent the variety form" with "a Hybrid, very fast-paced, high Energy [show] with sketches, impersonations, pet acts, and performers showcasing their talents". Hartman was to be the show's executive Producer and head Writer. Before production began, however, the network decided that variety shows were too unpopular and scrapped the series. In a 1996 interview, Hartman noted he was glad the show had been scrapped, as he "would've been sweatin' blood each week trying to make it work". In 1998, he admitted he missed working on SNL, but had enjoyed the move from New York City to Southern California.

Hartman's first starring film role came in 1995's Houseguest, alongside Sinbad. Other films included Greedy, Coneheads, Sgt. Bilko, So I Married an Axe Murderer, CB4, Jingle All the Way, Kiki's Delivery Service, and Small Soldiers, the last of which was his final theatrically released film. At the same time, he preferred working on television. His other television roles included appearances on episodes of The John Larroquette Show, The Dana Carvey Show, and the HBO TV film The Second Civil War as the President of the United States. He appeared as the kidnapper Randy in the third season cliffhanger finale of 3rd Rock from the Sun — a role reportedly written especially for him, but he died before filming of the concluding episode could take place. Executive Producer Terry Turner decided to recast the part, reshoot and air the finale again, noting: "I have far too much respect for [Hartman] to try to find some clever way of getting around this real tragedy." Hartman made a considerable amount of money from television advertising, earning $300,000 for a series of four commercials for the soft drink Slice. He also appeared in advertisements for McDonalds (as Hugh McAttack) and 1-800-Collect (as Max Jerome).

At the time of his death, Hartman was preparing to voice Zapp Brannigan, a character written specifically for him on Groening's second animated series Futurama. Even though the role was made for Hartman, he still insisted on auditioning and according to Groening, he 'nailed it'. After Hartman's death, Billy West took over the role of Brannigan. Though executive Producer David X. Cohen credits West with using his own take on the character, West later said that he purposely tweaked Zapp's voice to better match Hartman's intended portrayal. Hartman was also planning to appear with Lovitz in the indie film The Day of Swine and Roses, scheduled to begin production in August 1998.

Other causes for the incident were later suggested. Before committing the act, Brynn was taking the antidepressant drug Zoloft. A wrongful-death lawsuit was filed in 1999 by Brynn's brother, Gregory Omdahl, against Pfizer, the drug's manufacturer, and her child's Psychiatrist Arthur Sorosky, who provided samples of Zoloft to Brynn. Hartman's friend and former SNL colleague Jon Lovitz has accused Hartman's former NewsRadio co-star Andy Dick of re-introducing Brynn to cocaine, causing her to relapse and suffer a nervous breakdown. Dick claims to have known nothing of her condition. In 2006, Lovitz claimed that Dick had approached him at a restaurant and said, "I put the Phil Hartman hex on you; you're the next one to die." Lovitz responded by smashing Dick's face into the bar. The following year at the Laugh Factory comedy club in Los Angeles, Lovitz and Dick had a further altercation over the issue. Dick asserts that he is not at fault in relation to Hartman's death.

Laugh.com and Hartman's brother, John Hartmann, published the album Flat TV in 2002. The album is a selection of comedy sketches recorded by Hartman in the 1970s that had been kept in storage until their release. Hartmann commented: "I'm putting this out there because I'm dedicating my life to fulfilling his dreams. This [album] is my brother doing what he loved." In 2013, Flat TV was optioned by Michael "Ffish" Hemschoot's animation company Worker Studio for an animated adaptation. The deal came about after Michael T. Scott, a partner in the company, posted a hand-written letter he had received from Hartman in 1997 on the internet, leading to a correspondence between Scott and Paul Hartmann.

Hartman was widely mourned in Hollywood. NBC executive Don Ohlmeyer stated that Hartman "was blessed with a tremendous gift for creating characters that made people laugh. Everyone who had the pleasure of working with Phil knows that he was a man of tremendous warmth, a true professional and a loyal friend." Guttenberg expressed shock at Hartman's death, and Steve Martin said he was "a deeply funny and very happy person." Matt Groening called him "a master", and Director Joe Dante said, "He was one of those guys who was a dream to work with. I don't know anybody who didn't like him." Dan Snierson of Entertainment Weekly concluded that Hartman was "the last person you'd expect to read about in lurid headlines in your morning paper" and "a decidedly regular guy, beloved by everyone he worked with." In 2007, Entertainment Weekly ranked Hartman the 87th greatest television icon of all time, and Maxim named Hartman the top Saturday Night Live performer of all time.

In 2007, a campaign was started on Facebook by Alex Stevens and endorsed by Hartman's brother, Paul Hartmann, to have Hartman inducted to Canada's Walk of Fame. Amongst the numerous events to publicize the campaign, Ben Miner, of the Sirius XM Radio channel Laugh Attack, dedicated the month of April 2012 to Hartman. The campaign ended in success and Hartman was inducted to the Walk of Fame on September 22, 2012, with Paul accepting the award on his late brother's behalf. Hartman was also awarded the Cineplex Legends Award. In June 2013, it was announced that Hartman would receive a star on the Hollywood Walk of Fame, which was unveiled on August 26, 2014. Additionally, a special prize at the Canadian Comedy Awards was named for Hartman. Beginning with the 13th Canadian Comedy Awards in 2012, the Phil Hartman Award was awarded to "an individual who helps to better the Canadian comedy community." In 2015, Rolling Stone magazine ranked Hartman as one of the top-ten greatest Saturday Night Live cast members throughout the show's forty-year history, coming in seventh on their list of all one-hundred-and-forty-one players.

Ken Tucker summarized Hartman's comedic style: "He could momentarily fool audiences into thinking he was the straight man, but then he'd cock an eyebrow and give his voice an ironic lilt that delivered a punchline like a fast slider—you barely saw it coming until you started laughing." Hartman claimed that he borrowed his style from actor Bill Murray: "He's been a great influence on me – when he did that smarmy thing in Ghostbusters, then the same sort of thing in Groundhog Day. I tried to imitate it. I couldn't. I wasn't good enough. But I discovered an element of something else, so in a sick kind of way I made myself a career by doing a bad imitation of another comic."