Born in Brighton, Janus's uncle was football legend George Best. Her father left the family when she was three, and after her mother married a doctor the family moved to Edinburgh, Scotland. She has also lived on the QE2 for a short time.

Aged fifteen she left home as a self-confessed wild child for a life in squats, drinking and taking drugs. She eventually turned her life around, and attended the Sylvia Young Theatre School in London.

Janus first came to prominence in 1991, when she won the annual A Song For Europe competition to represent the United Kingdom in the Eurovision Song Contest. She finished equal tenth and the song, "A Message to Your Heart", peaked at number 30 on the UK Singles Chart.

She developed her first love of acting, but was often initially mistaken for fellow actress Sarah Alexander. Janus appeared in episodes of The Bill and Press Gang; in 1993 she appeared in "Demob" alongside Griff Rhys Jones and Martin Clunes; and took to the West End stage, replacing Debbie Gibson as Sandy in "Grease."

In 1994 she landed the part of Mandy in the cult BBC Two sitcom Game On alongside Ben Chaplin. Showing a natural comic touch, she stayed on the show until it finished in 1998. In between series of Game On, she had a part in the BBC One drama Pie in the Sky from 1995 until 1996.

Janus then starred in The Grimleys with Jack Dee and movie Up n Under with Neil Morrissey. In 1998 she starred in the ITV1 series Liverpool 1 alongside her future boyfriend Mark Womack. She then returned to comedy in the ITV sitcom Babes in the Wood. Co-starring another Sylvia Young Theatre School student, Denise van Outen, it was critically panned despite respectable ratings, and Janus left after the first series.

In 1999 she starred as Cinderella in the ITV Christmas pantomime, with Harry Hill, Alexander Armstrong, Frank Skinner, Paul Merton, Ronnie Corbett and many more.

In 2002 she returned to TV screens in the BBC One drama Strange and after that series ended in 2003, she has had roles in the films The Baby Juice Express and Dead Man's Cards.

2005 saw Samantha directing an anti-bullying music video for Liverpool band Just 3, entitled Stand Tall. The video starred Liverpool FC footballer Jamie Carragher and herself. The video was heavily backed by Childline ambassador Esther Rantzen.

A popular choice for photo shoots in men's magazines such as FHM, she came 21st in the FHM 100 Sexiest Women in the World 1998 poll and 41st in the same listing in 1999.

In 2006, Janus starred in the BBC One comedy Home Again as Ingrid, and as Miss Adelaide in the West End production of the musical Guys and Dolls at London's Piccadilly Theatre, initially alongside Patrick Swayze and then Miami Vice star Don Johnson. She reprised the role at the Sunderland Empire alongside Alex Ferns in 2007.

On 11 May 2007, it was announced that Janus would be joining EastEnders as Ronnie Mitchell, a cousin of the Mitchell Brothers.