He first came to widespread attention in the late 1980s and early 1990s for his roles on the sketch comedy show Saturday Night Live, afterwards going on to motion pictures, frequent roles on the animated The Simpsons, and a major part as "Bill McNeal" on the sitcom NewsRadio.

Hartman was born in Brantford, Ontario, Canada to Doris M. and Rupert L. Hartmann. His family emigrated to the United States in the 1950s, and Hartman attended Redondo Union High School and Cal State Northridge in Los Angeles, California, becoming a naturalized U.S. citizen in the early-1990s. The exact timing of his switch from "Hartmann" to "Hartman" is unknown, but all of his acting credits after 1986 were billed under the surname "Hartman".

Looking for what he described as "a psychological release valve", he joined the California-based comedy group The Groundlings in 1975 . Hartman met comedian Paul Reubens while working with the group and the two became friends, often writing and working on material together. One such collaboration was the character of Pee-wee Herman and the script of the feature film Pee-wee's Big Adventure. Hartman also met Jon Lovitz while with The Groundlings. Hartman was fluent in German, and a video circulated once of him doing a standup routine in the language.

Hartman worked part time as a graphic artist, including designing album covers for popular rock bands. Hartman's covers include:

In 1986, Hartman joined the cast of NBC's popular variety show Saturday Night Live and stayed for eight seasons, which was a record at the time. Hartman was known for his impressions, which included Ronald Reagan, Charlton Heston, Frank Sinatra, Telly Savalas, Ed McMahon, Michael Caine, Jack Nicholson, Barbara Bush, Burt Reynolds, Phil Donahue, and former president Bill Clinton, which was perhaps his best-known impression. His other Saturday Night Live characters included Frankenstein's monster and Unfrozen Caveman Lawyer. He returned twice to host the show following his 1994 departure.

Also in 1986, Hartman reprised his role as Captain Carl, one of Pee-Wee Herman's close friends and famed sea captain in the first season of Pee-Wee's Playhouse. He left after the 13th episode. He also lent his voice to the cartoon version of Dennis the Menace, playing Henry Mitchell (Dennis' father).

From 1991 to 1998, Hartman also provided the voices for a number of characters on the popular animated series The Simpsons, including dubious attorney Lionel Hutz, B-movie actor Troy McClure, Bart's "bigger brother" Tom, and slippery monorail shyster Lyle Lanley. In the episode "Selma's Choice", he lent his voice to five different characters, including McClure and Hutz.

In 1994, Hartman left SNL. His last scene on Saturday Night Live consisted of him consoling Chris Farley (who was dressed as his Matt Foley character). Hartman's next project was to be a self-titled network variety show; after Dana Carvey announced plans for his own variety show, Hartman instead cast his eye on a more standard sitcom. In 1995, he became one of the stars of the NBC sitcom NewsRadio, where he portrayed fatuous radio news anchor Bill McNeal. Many have attributed the cancellation of the show to Hartman's death, citing that the humor was thrown off balance despite the casting of Lovitz (who replaced Hartman). Ironically in two episodes of NewsRadio Hartman's character was one of only two to survive tragedies. He makes mention several times of a girlfriend who is unstable and who tries to kill him. He also believes he is being stalked by strangers including a man dressed as Santa Claus. In one episode a psychic tells Bill the date of his death, his 82nd birthday.

Hartman was appearing in a cliffhanger on the series 3rd Rock from the Sun at the time of his death. His character did not appear in the second episode of the cliffhanger.

Hartman's filmography includes often secondary or supporting roles in such feature films as Pee-Wee's Big Adventure, Greedy, Houseguest, Coneheads, Three Amigos, Stuart Saves His Family (voice only), Sgt. Bilko, So I Married an Axe Murderer, CB4, Jingle All The Way and Small Soldiers, the last of which would become his final silver screen appearance and was thus dedicated to him. His last role was in the English version of Hayao Miyazaki's Kiki's Delivery Service, where he provided the voice of Jiji the cat. The English dub of the movie was also dedicated to his memory.

Hartman married Gretchen Lewis in 1970, and divorced at an unknown point, before 1982. In 1982 he married his second wife Lisa Strain, the marriage lasted three years with the couple divorcing in 1985. Hartman married Brynn Hartman on November 25, 1987, Brynn having become pregnant on the pairs third date. Together they had two children, Sean Edward Hartman (born 1989) and Birgen Anika Hartman (born 1992). Brynn was immensely jealous of Hartman's success, and often speculated that Hartman was together with his Saturday Night Live co-star Jan Hooks, which lead to their marriage problems and then the pairs' death. The children are now being raised in Eau Claire, Wisconsin by Brynn's sister Katharine Wright and her husband Mike, who have no children of their own. As guardians of Sean and Birgen, the pair were awarded $50,000 per Phil Hartman's will. In May 2000, the Hartman children were awarded more than $10,000 each in a wrongful death suit. Hartman's will stipulated that each child will receive a third of their inheritance when they reach age 25, or obtain a bachelor's degree from a four-year accredited university. They get half their inheritance when they turn thirty, and the remainder of Hartman's assets when they turn 35. The total amount of Hartman's estate was estimated at $1.23m.

On May 28, 1998, Hartman was murdered by Brynn Hartman in his Encino, California home. While he slept, Brynn entered his bedroom with a revolver and shot him multiple times in the head region which killed him. Brynn was, at the time, drunk. She then drove to the home of her friend Ron Douglas and confessed what she had just done, after which she fainted. After she awoke, the pair drove back to Hartman's house in separate cars, where Douglas saw Hartman's body. Douglas then phoned the emergency services, admitted to the call operator that he initially did not believe Brynn's story until he had seen Phil's body. On arrival, the police attempted to remove the Hartmans' two young children and Douglas from the premises. Whilst this was happening, Brynn entered the bedroom and committed suicide by shooting herself once through the temple. At the time, an unnamed neighbour of the Hartman's told a CNN reporter that the couple had been experiencing marital problems: "It's been building, but I didn't think it would lead to this." However, Steve Guttenberg commented that the pair "a very happy couple, and they always had the appearance of being well-balanced." Phil Hartman directed in his will that his body be cremated, and his ashes were scattered over Santa Catalina Island's Emerald Bay.

Los Angeles police stated that Hartman's murder was due to a "domestic discord" between the couple. The antidepressant drug Zoloft, which Brynn Hartman was known to have been taking at the time of the murder, was cited as the cause of her actions. A wrongful-death suit was filed by Gregory Omdahl against the drugs' manufacturer Pfizer Inc., as well as Brynn Hartman's psychiatrist Arthur Sorosky who gave her the product.

Hartman was mourned greatly in Hollywood. NewsRadio produced a special episode where the cast tearfully mourned the death of Hartman's on-screen counterpart. Jon Lovitz joined the show in his place and stayed with it until its ultimate cancellation. Lovitz had been a Saturday Night Live cast member alongside Hartman for four seasons, and had also worked with Hartman when they both had cameos in the 1986 comedy Three Amigos. Lovitz and Hartman's former SNL colleagues—Jan Hooks, Kevin Nealon, Victoria Jackson, and Mike Myers—also gave him an emotional remembrance at the SNL 25th Anniversary show in 2000. Hartman's nickname, coined by Adam Sandler, according to Jay Mohr's book Gasping for Airtime was "Glue", because of his way of working well with everyone, and keeping many episodes together. Hartman was widely praised for his efforts to help new writers and castmembers, as well as his professionalism on and off the set. Many people gave tibutes to Hartman. Don Ohlmeyer said that Hartman "was blessed with a tremendous gift for creating characters that made people laugh. But more importantly, 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." Steve Guttenberg expressed his shock at Hartman's death, and Steve Martin called him "a deeply funny and very happy person".

Out of respect, the writers on The Simpsons retired Hartman's characters, rather than finding another voice actor. The episode "Bart the Mother" marked his final appearance on the show, and was dedicated to him. On audio commentary tracks accompanying The Simpsons DVD releases, the appearance of a Hartman character uniformly evokes heartfelt praise from the show's production staff. A number of major Simpsons players talk at length about their experience of Hartman as a rare combination of professionalism, effortless comedic facility, friendliness, and decency. Bill Oakley and Josh Weinstein stated that they used Hartman as much as physically possible in their seasons, as they were such big fans of him. As his voice acting could produce "the maximum amount of humour" out of any line he was given. Before his death, Hartman had expressed an interest in making a live action film about Troy McClure, with many of The Simpsons production staff stating that they would have loved to help create it.[10]

At the time of his death, Hartman was preparing to voice Zapp Brannigan, a character on The Simpsons creator Matt Groening's other animated series Futurama, a part that Groening had written specifically for him.[11] After he died, the lead character in Futurama Philip J. Fry, was named in Hartman's honor.[11] Billy West took over the role. West's original audition characterization formed the basis for Brannigan's voice, but by coincidence his portrayal bears many similarities to Hartman's own vocal stylings. West later admitted that he purposefully tweaked Zapp's voice to better match Hartman's portrayal out of respect.[11]

Hartman was posthumously nominated for an Emmy Award for his portrayal of Bill McNeal in NewsRadio,[12] but lost out to actor David Hyde Pierce. Upon learning Hartman did not win the award, NewsRadio co-star Dave Foley joked "What's this guy gotta do to win an Emmy?".