Fleming was born Marilyn Fleming in New Liskeard, Ontario, Canada. She appeared in minor roles in six films between 1965 and 1976, during which time she became acquainted with Marx and moved into his house.
Fleming's influence on Marx was controversial. Many close to him admitted that she did much to revive his popularity; these efforts included a series of one-man shows, culminating in a sold-out performance at Carnegie Hall which was released on a best-selling record album and an honorary Academy Award he received in 1974. Groucho's comic persona (and real-life behavior, as well) included a playful but wolfish pursuit of women. As such, observers felt the apparent relationship with a young starlet boosted Groucho's ego, adding to his vitality. Others, including Marx's son,Arthur, described her in Svengali-esque terms, accusing her of exploiting an increasingly senile and frail Marx in pursuit of her own stardom.
In the years leading up to Marx's death in August 1977, his heirs filed several lawsuits against Fleming. One allegation leveled against Fleming was that she was determined to sell Marx's favorite car, a Cadillac, against his wishes. When Marx protested, it was said, Fleming threatened, "I will slap you from here to Pittsburgh." Another allegation had her dancing nude around Marx, fondling herself and asking "Don't you wish you could have some of this?" Many people close to Marx believed Fleming was abusive towards him. Arthur wanted temporary conservatorship of his father, and took Fleming to court. According to the book Raised Eyebrows by Groucho's secretary Steve Stoliar, Fleming had several personal problems; he stated in his book that she used drugs, hadmood swings, and was given to inappropriate outbursts, both in public and in private.
One bizarre incident involved Fleming hiring private detectives to search the Marx home for listening devices that she thought had been installed at Arthur Marx's behest. The purpose of hidden surveillance was for him to gain leverage in their court battle. The detectives found no listening devices, but did find bag of used hypodermic syringes and vials of pharmaceutical grade tranquilizers. These were controlled narcotics that could only be administered by medical professionals and had not been prescribed to Groucho or anyone else who would have had access to the property. Fleming directed the detectives to dispose of the syringes and vials in a storm drain at the edge of the property. Instead, the private detectives took them to the local police to report what they had found. Very soon, the media erupted with lurid stories, reaching the inference that Groucho was being drugged against his knowledge or that of his doctors.
The court battles dragged into the early 1980s, but judgments were eventually reached in favor of Arthur Marx, ordering Fleming to repay $472,000 to the Marx estate.
Fleming's mental health deteriorated in the 1990s. She was arrested once in the Los Angeles area on a weapons charge, and spent much of the decade in and out of commitments to various psychiatric facilities. She was also reportedly impoverished and homeless in her final years, living on the streets of Hollywood and Beverly Hills.
Fleming committed suicide in 2003 by shooting herself.