The screenplay upon which the film was based was written by Colin Higgins, and published as a novel in 1971. The movie was shot in the San Francisco Bay Area. Harold and Maude was also a play on Broadway for some time. A French adaptation for television, translated and written by Jean-Claude Carrière, appeared in 1978. It was adapted for the stage and performed in Québec, starring Roy Dupuis.

The film is ranked number 45 on the American Film Institute's list of 100 Funniest Movies of all Time, and was selected for preservation in the National Film Registry of the Library of Congress in 1997 for being "culturally, historically, or aesthetically significant." The film was a commercial flop in its original release, and critical reception was extremely mixed. However, it has since developed a large cult following.

Harold Chasen (Bud Cort) is a 19-year-old boy obsessed with death. He regularly stages elaborate suicides, attends funerals, and drives a hearse, all to the chagrin of his mother, socialite Mrs. Chasen (Vivian Pickles). At funeral services for a total stranger, Harold meets Maude (Ruth Gordon), a 79-year-old woman who shares Harold's hobby of attending funerals for fun. He is entranced by her quirky outlook on life, which is bright and excessively carefree in contrast with his own morbidity. The pair form a bond, and Maude slowly shows Harold the pleasures of art and music (Harold is taught to play banjo), and teaches him how to "[make] the most of his time on earth." Meanwhile, Harold's mother determines, much against Harold's wishes, to find him a wife to settle down with. One by one, Harold frightens and horrifies each of his appointed dates by appearing to commit gruesome acts such as self-immolation, self-mutilation, and seppuku.

As they become closer, Harold announces that he will marry Maude, resulting in disgusted outbursts from his family. Maude's 80th birthday arrives, and Harold throws a surprise party for her. As the couple dance, Maude tells Harold that she "couldn't imagine a lovelier farewell." He immediately questions Maude as to her meaning, and she reveals that she has purposely taken an overdose of sleeping pills and will be dead by midnight. She restates her firm belief that 80 is the proper age to die.

Harold rushes Maude to the hospital, where she is treated unsuccessfully and dies. In the final sequence, Harold's car is seen going off a seaside cliff, but after the crash, the final shot reveals Harold standing quite calmly atop the cliff, holding his banjo. After gazing down at the wreckage, he dances away, picking out on his banjo Cat Stevens' "If You Want to Sing Out, Sing Out".

Hal Ashby, the film's director, shared certain ideals with the era’s youth culture, and in this film he contrasts the doomed outlook of the alienated youth of the time with the hard-won optimism of those who endured the horrors of the early 20th century, contrasting nihilism with purpose. Maude's past is revealed in a glimpse of the concentration camp ID number tattooed on her arm as well as her talk with Harold about using an umbrella to defend herself from thugs at political meetings before moving to America.

Harold is part of a society in which he is of no importance; existentially, he is without meaning. Maude has survived and lives a life rich with meaning and deliberate choice. It is in this existential crisis, shown against the backdrop of the Vietnam War, that we see the differences between one culture, personified by Harold, handling a meaningless war, while another has experienced and lived beyond a war that produced a crisis of meaning.

Harold tells Maude when they are talking candidly at her house he explains how he has "died a few times." Harold describes that when he was at boarding school, he set his chemistry lab on fire. Escaping the fire through a hole in the floor, Harold went home. When the police came to his house, Harold watched as they told his mother that he had died in the fire and saw his mother collapse into the policemen's arms. As he reaches this part of the story, Harold bursts into tears and declares, "I decided then I enjoyed being dead."

Throughout the movie, Harold appears to "die" a total of seven to eight times. He tells his psychologist that he has made similar attempts 15 times, which he calls a rough estimate.

Harold and Maude is #45 on the American Film Institute’s list of 100 Years... 100 Laughs, the list of the top 100 films in American comedy. The list was released in 2000. Two years later, AFI released the list AFI's 100 Years... 100 Passions honoring the most romantic films for the past 100 years, Harold and Maude ranked #69. Entertainment Weekly ranked the film #4 on their list of “The Top 50 Cult Films.”