Born in Brighton, Womack is the niece of Angie Best, a former wife of George Best. Her cousin is Calum Best, son of Angie and George. She has a younger sister, Zoe. Her father Noel left the family when she was three, and soon after her mother Diane married a doctor and the family moved to Edinburgh. Janus lived on the QE2 with her grandmother for a short time. On 23 August 2009, her 60 year old estranged father was found hanged in the bathroom of his council flat, where he lived alone. It is believed he committed suicide.
Aged fifteen she left home as a self-confessed wild child for a life in squats, drinking, smoking and taking drugs. She eventually turned her life around, and attended the Sylvia Young Theatre School in London.
The actress 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.
Janus developed her first love of acting, appearing 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.
She then starred in the pilot for The Grimleys with Jack Dee, and the movie Up 'n' Under with Neil Morrissey. In 1998 she starred in the ITV1 series Liverpool 1 alongside her future husband 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.
In 2005 she directed 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, she 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 she would be joining EastEnders as Ronnie Mitchell, a cousin of the Mitchell brothers. She had previously appeared in the soap in a minor role as a girlfriend of Simon Wicks.
In 2009, Janus took her husband's surname of Womack and began to use it professionally in her role on EastEnders'
They married on 16 May 2009 while Mark has another son from a previous marriage, Michael Womack Their son Benjamin Thomas Womack was born in February 2001 (after having him Janus took two years off to enjoy motherhood) and daughter Lily-Rose Womack and her husband are friends with Scott Maslen and his wife Estelle Rubio. Maslen is godfather to Samantha's two children (and she is godmother to Maslen's son, Zak) and they often go on holiday together with their children, which Womack revealed during an appearance on Friday Night with Jonathan Ross in January 2008.Sam is also best friends with co-star Rita Simons, who plays her sister Roxy Mitchell In the autumn of 2007, Maslen joined the cast of 'EastEnders as the character Jack Branning the brother of Max Branning who quickly takes a shine to Ronnie. This was something that both Maslen and Womack were initially uncomfortable with when they read it in the scripts, Maslen commenting in various interviews that having to kiss Womack was like having to kiss his sister