After leaving high school to pursue an entertainment career in New York, Elliot toured in the musical The Music Man in 1962, but lost the part of Miss Marmelstein in I Can Get It for You Wholesale to Barbra Streisand. Elliot would sometimes sing while working as a cloakroom attendant at The Showplace in Greenwich Village, but she did not pursue a singing career until she moved to the Washington, D.C. area to attend American University (not Swarthmore College, as mentioned in the biographical song "Creeque Alley").

Elliot was married twice, the first time in 1963 to James Hendricks, her group mate in the Big 3 and the Mugwumps. This was reportedly a platonic arrangement to assist him in avoiding being drafted during the Vietnam War; the marriage reportedly was never consummated and was annulled in 1968. In 1971, Elliot married Journalist Donald von Wiedenman, heir to a Bavarian barony. Their marriage ended in divorce after a few months.

Tim Rose left the Big 3 in 1964, and Elliot and Hendricks teamed with Canadians Zal Yanovsky and Denny Doherty to form the Mugwumps. This group lasted eight months, after which Cass performed as a solo act for a while. In the meantime, Yanovsky and John Sebastian co-founded the Lovin' Spoonful, while Doherty joined the New Journeymen, a group that also included John Phillips and his wife Michelle. In 1965, Doherty persuaded Phillips that Elliot should join the group, which she did while she and the group members were vacationing in the Virgin Islands.

Elliot gave birth to daughter Owen Vanessa Elliot on April 26, 1967. She never publicly identified the father, but many years later, Michelle Phillips helped Owen locate her biological father. After Elliot's death, her younger sister, Leah Kunkel (then married to Los Angeles-based session Drummer Russ Kunkel), received custody of Owen, then seven years old, and raised her along with her own son, Nathaniel. Owen grew up to become a singer as well and toured with Beach Boys member Al Jardine.

In October 1968, Elliot made her live solo debut headlining in Las Vegas at Caesars Palace, scheduled for a three-week engagement at $40,000 per week (US$281,493 in 2017 dollars), with two shows per night. Elliot went on a six-month long crash diet before the show, losing 100 of her 300 pounds. According to Elliot, the weight loss led to a stomach ulcer and throat problems, which she treated by drinking milk and cream (and regaining 50 lbs. in the process).

Elliot appeared in two television variety specials: The Mama Cass Television Show (ABC, 1969) and Don't Call Me Mama Anymore (CBS, 1973). She was a regular guest on TV talk shows and variety shows in the early 1970s, including The Mike Douglas Show, The Andy Williams Show, Hollywood Squares, The Johnny Cash Show, The Smothers Brothers Comedy Hour, and The Carol Burnett Show, and was a guest panelist for a week on the game show Match Game '73. She guest-hosted for Johnny Carson on The Tonight Show and appeared as a guest on the show 13 other times. She also appeared on and co-hosted The Music Scene on ABC and was featured on the first The Midnight Special on NBC.

Throughout the early 1970s, Elliot continued her acting career, as well. She had a featured role in the movie Pufnstuf (1970) and made guest appearances on TV's The New Scooby-Doo Movies, Young Dr. Kildare, Love, American Style, and The Red Skelton Show, among others.

The Mamas & the Papas continued to record to meet the terms of their record contract. Their final album was released in 1971.

She performed the title song "The Good Times Are Comin'" during the opening sequence of the 1970 film Monte Walsh, starring Lee Marvin and Jack Palance. In 1972, she made three appearances on the variety series The Julie Andrews Hour. Her final appearance on the show was the Christmas installment that aired on Wednesday, December 20, 1972. In December 1978, four years after Elliot's death, the episode was rebroadcast on syndicated stations as a Christmas special called Merry Christmas With Love, Julie. However, all of Elliot's solos were deleted from the syndicated prints. In 2009, a complete videotape of The Julie Andrews Hour Christmas Show was donated to The Paley Center For Media in New York, with all of Elliot's numbers intact.

Elliot's recording of "Make Your Own Kind of Music" is featured prominently in several episodes of seasons 2 and 3 of Lost as well as season 8, episodes 2 and 9 of Dexter (the later one also uses the title as the episode's title). Her recording of "It's Getting Better" was featured in a season 4 episode of Lost. She also provided the voice for her appearance on a 1973 episode of the New Scooby Doo Movies, "The Haunted Candy Factory".

In July 1974, Elliot performed two weeks of concerts as a solo performer at the London Palladium. Many claimed that all of these shows were sold out, but she was often playing to a less-than-full house after the earliest dates. She made an international call to Michelle Phillips after the final concert on July 28. Phillips said later that Elliot sounded elated that she had received standing ovations each night.

The Crosby, Stills & Nash Daylight Again video released in 1982 was dedicated to Cass Elliot.

David Crosby published a memoir in 1988, saying the following about his use of heroin with Elliot:

Elliot was the subject of a 2004 stage production in Dublin, The Songs of Mama Cass, with Kristin Kapelli performing main vocals.

The Crosby, Stills & Nash Greatest Hits album released in 2005 was dedicated to Cass Elliot.

She may have retired for the evening immediately after this telephone conversation, but Debbie Reynolds claimed in her 2013 book Unsinkable: A Memoir that she and her children Carrie Fisher and Todd Fisher saw Elliot at a party that night at the London home of Mick Jagger. Reynolds noticed that when Elliot left the party saying she was headed to where she was staying, she was not accompanied by anyone.

An oft-repeated urban legend is that Elliot choked to death on a ham sandwich. The story spread soon after the discovery of her body and was based on speculation in the initial media coverage. A 2014 article in Haaretz identified the person who started the false rumor as follows: "Unfortunately, the first Doctor [in London] who examined her speculated to the press about the cause of death, and that’s the version that stuck." An autopsy had not been performed when the physician was quoted, and London police told reporters that a partially eaten sandwich found in her room might have been relevant to the cause of death. The post-mortem found that Elliot had died of heart failure, and no food was present in her windpipe, yet the false story has persisted ever since.

A cover version of Make Your own Kind of Music has been recorded by Paloma Faith which is featured in the Skoda advert: Driven By Something Different(2018), for their Karoq model car. Paloma Faith's cover version is also being released as a single