Marshall is multiracial; her father is Caucasian and her mother is Caribbean-Canadian.
Marshall studied music extensively in childhood, including at the Royal Conservatory of Music in Toronto. While performing on the Queen Street West bar scene in her teens, she met guitarist Jeff Healey, who was struck by her powerful voice and took her on tour. She was offered a record deal by Columbia Records in 1991, but chose to wait a few years.
1995, Marshall signed to Sony Records, and released her self-titled debut album that year. The album was a massive hit in Canada, spawning six Top 40 hits -- "Let It Rain", "Beautiful Goodbye", "Dark Horse", "Fall From Grace", "Sitting on Top of the World" and of course "Birmingham", which remains her biggest hit internationally.
In 1999, she released her follow-up album, Tuesday's Child. It followed in the same vein as her debut, with a mix of soulful pop songs and ballads, characterizing her powerful voice.
Her song "Ride" would be featured in the The Replacements and on its soundtrack. The song was noticeable in the scene when Shane Falco (played by Keanu Reeves) first noticed Annable (played by Brooke Langton) when he was at practice for the first time, showing off a love scene. Another one of Marshall's Canadian hits, "Believe In You," was featured on one episode of Touched By An Angel. Richie Sambora of Bon Jovi was the guest guitarist on the track call "Why Don't You Love Me" which never became a hit.
In 2001, she released her third album, Everybody's Got a Story. It marked a change in style and sound for Marshall, with a noticeable R&B influence. She had hits with "Sunday Morning After," "Marry Me," "Everybody's Got A Story," "Double Agent"
In 2003, she released a greatest hits album entitled Intermission.