The Actress first came to prominence in March 1991 aged 18, when she won the annual A Song for Europe competition to represent the United Kingdom in the Eurovision Song Contest. She finished joint tenth and the song, "A Message to Your Heart", peaked at number 30 on the UK Singles Chart. Despite her achievements, her entry into the Eurovision is frequently remarked upon. She has said regarding the experience that her failure in the contest devastated her at the time and thought it would spell the end of her career. She has also called her appearance 'ridiculous', in that she was dressed in a pink mini-dress while singing about starving children and says the lyrics of the song are burned into her memory forever.

Womack developed her first love of acting, appearing in episodes of The Bill and Press Gang; in 1993 she appeared in Demob with Griff Rhys Jones and Martin Clunes; and took to the West End stage, replacing Debbie Gibson as Sandy in Grease.

From 1994 to 1998 Womack played Mandy in the BBC Two sitcom Game On opposite Ben Chaplin (later replaced by Neil Stuke). She remained on the show until its end in 1998. In between series of Game On, she had a part in the BBC One drama Pie in the Sky from 1995 to 1996 as Nicola Dooley, a waitress.

Janus married Italian Junfan Mantovani in 1997 after a long relationship. The relationship broke down and the couple divorced in 1998.

Womack then appeared in the pilot for The Grimleys with Jack Dee, and the film Up 'n' Under with Neil Morrissey. In 1998 she starred in the ITV1 series Liverpool 1 with her Future husband Mark Womack. She then returned to comedy in the sitcom Babes in the Wood co-starring another Sylvia Young Theatre School student, Denise van Outen, it was critically panned despite respectable ratings, and Womack left after the first series.

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

In 2002 she returned to TV in BBC One drama Strange and she appeared in films The Baby Juice Express, Lighthouse Hill 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 her and Liverpool F.C. footballer Jamie Carragher. 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 poll in 1998 and 41st 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 with Patrick Swayze and then Miami Vice star Don Johnson. She reprised the role at the Sunderland Empire opposite Alex Ferns in 2007.

In 2007 Womack won "substantial" damages from the Sunday Sport newspaper over a story headlined "Sam's sex and drugs shame". She sued the Sport over an article published on 22 July, which claimed she had "coke fuelled threesomes" and took part in a "three-hour drug fuelled lesbian orgy"

In April 2008, several Future scenes featuring Ronnie had to be scrapped after Womack broke her shoulder when she tripped and fell down the stairs at her home while rushing to get ready for work.

In 2009 Womack took her husband's surname and began to use it professionally. In October 2017, Womack spoke of her father's 2009 suicide stating that she initially blamed herself as she ignored his calls because he had ‘behaved badly’ at her wedding earlier that year.

In November 2011, it was reported that Womack was in the running, alongside Dannii Minogue to become a judge on the 2012 series of Britain's Got Talent. However, it was later confirmed that the judging panel for series six would be Simon Cowell, Alesha Dixon, Amanda Holden and David Walliams. Womack has since confirmed that she was offered a place on the judging panel and that she had a meeting with Cowell in Los Angeles. She has said that although she was grateful, she rejected the offer as "it’s just not me".

In 2012 Womack appeared in series nine of the BBC's Who Do You Think You Are? The episode revealed her descent on her father's side from two Musicians, her great grandfather Alexander Cunningham Ryan, his partner's father Pierre François Odilon Garraud, & Pierre's partner, an Actress, Jessie Ryder.

In May 2013, it was confirmed that Womack would return to EastEnders as Ronnie Mitchell. In March 2013, Womack starred opposite her husband on stage in the world premiere of Hope at the Royal Court Theatre in Liverpool. In 2015, she appeared in the hit Hollywood film Kingsman: The Secret Service as lead actor Taron Egerton's character's mother. This was Womack’s first main role in such a high profile film.

Janus lived on the QE2 with her Choreographer grandmother, Sheila Holt (a former Actress and Choreographer to Magician David Nixon for a short time). Womack has a younger half-sister, Zoe. During an appearance on Big Star's Little Star in 2014, it was revealed that she attended the same school as members of All Saints before going to the Sylvia Young Theatre School in London.

In September 2015, it was reported that she was "living in fear" and had alerted the police that she was being stalked. The stalker was given a suspended-sentence in December, the same year.

On 13 August 2016, it was reported that Womack had been offered a "dramatic ending" to her character's EastEnders storylines which she had accepted, with her exit due to be broadcast in early 2017. Her character Ronnie Mitchell died by trying to save Roxy Mitchell from drowning and drowned herself. Her last appearance was on 19 January 2017 when Ronnie reappeared as a corpse having been killed-off on the episode broadcast on 1 January 2017. Womack has since spoke out about Ronnie’s death saying that her exit was ‘short-sighted’ and that she hates the idea of not going back, despite originally defending the boss in earlier interviews.

In May 2017, Womack endorsed Labour Party leader Jeremy Corbyn in the 2017 UK general election. She tweeted: "The legislation set out by @jeremycorbyn is everything we need to believe in. It's time to pull together and vote. Stop the madness #️."

In March 2018, Womack opened up about a battle with anxiety and insomnia, stating that it was at its worst when filming Mount Pleasant and that she used to get hardly any sleep or none at all. Womack has said to cope with this, she does things like yoga which help.