Cowsill began her musical career with the Cowsills in 1967; she is the youngest member of the Cowsills and the only daughter of parents Bud and Barbara Cowsill. She made her debut on We Can Fly, the Cowsills’ second MGM Records album, released in early 1968. Her debut solo vocal was a song called Ask the Children, featured in the Cowsills’ third MGM album, Captain Sad And His Ship Of Fools. Her contribution to the Cowsills’ backing vocals made her, upon her ninth birthday, the youngest person to be directly involved in a top ten hit record when “Indian Lake” made the Top 10 in the early summer of 1968.

In 1969 she contributed to the vocals in what would become the Cowsills’ biggest hit, Hair. She became known for her performance of the line, “and spaghetti’d” which she sang with a squeakiness in her voice that she still uses when she performs the song live.

Susan was initially relegated to playing the tambourine, but by the time she left the group in 1971 (shortly after the release of their London Records album On My Side) she had learned to play other instruments; in an episode of the short-lived Barbara McNair Show she was seen playing bass guitar.

Cowsill signed briefly with Warner Bros. Records in 1976, releasing two singles, including revival of 1971 Sixto Rodriguez song “I Think of You”. Beginning in the early ’80s she worked as a backing vocalist for varying artists including Dwight Twilley, the Smithereens, Carlene Carter, Mike Zito, and Hootie & the Blowfish. During this time her songwriting skills blossomed, and several of her songs have been covered by other artists.

In 1978, she reunited with the Cowsills (without Bill and Barbara) to work on an album of new songs. The album, tentatively titled Cocaine Drain, was produced by Chuck Plotkin, but was not released until 2008. She again reunited with brothers Bob, Paul and John as the Cowsills in the 1990s, to work on another album of original songs. The album, Global, was released in 1998.

By the early 1990s, she had developed an affinity for Americana-style music, which in 1991 led to her joining the Continental Drifters, further honing her songwriting talents. She occasionally appeared in a duo with bandmate Vicki Peterson, calling themselves the Psycho Sisters. (Peterson subsequently married John Cowsill, currently playing drums for the touring version of The Beach Boys.) Cowsill permanently relocated to New Orleans, Louisiana by 1993.

She married fellow band member and drummer Russ Broussard in July 2003. She has one daughter, Miranda Holsapple, from her marriage to musician Peter Holsapple. Her sister-in-law is The Bangles guitarist Vicki Peterson.

In 2004, on the heels of a rare Christmas snowfall in New Orleans, Cowsill wrote and recorded Crescent City Sneaux, contrasting the peace of that snowfall with the disaster of Hurricane Katrina and has been described as an “anthem” for survivors of the hurricane.

By 2005, Cowsill had released her first solo album, Just Believe It, on her own Blue Corn indie label.

The Susan Cowsill Band’s “Covered In Vinyl” performances have featured classic rock albums played live in their entirety. Two album releases have been compiled from these shows: Live at Carrollton Station: Covered In Vinyl Series Vol. 1, released in 2007, and Vol. 2 in 2009. A portion of proceeds from CD sales have benefited New Orleans charities. Two digital albums from subsequent performances, CIV: Duets and CIV: Neil Diamond (recorded in 2010) were made available in 2012. The CIV band has included New Orleans guitar virtuoso Jimmy Robinson (Woodenhead, Twangorama), Pete Winkler (Motorway), Caleb Guillotte (Deadeye Dick), Derek Huston, Paul Sanchez, and many others.

Cowsill’s second solo album, Lighthouse, was released in 2010 with support from the New Orleans musicians’ organization Threadheads. It is a concept album in which she reflects on her losses, mainly through Hurricane Katrina and the deaths of brothers Barry and Bill. The album features harmonies from her surviving brothers (Bob, Paul and John) as well as appearances by Jackson Browne and Vicki Peterson, and was released May 18, 2010.

In 2012, Cowsill, Freedy Johnston, and Jon Dee Graham, working together as the Hobart Brothers and Lil’ Sis Hobart, released a collaborative album entitled At Least We Have Each Other. In August 2014, after playing together for more than 20 years, Cowsill and Peterson released their first album as the Psycho Sisters, entitled Up on the Chair, Beatrice.