Baron's film appearances include roles in The Small World of Sammy Lee (1963), Hot Millions (1968), Mrs. Brown You've Got a Lovely Daughter (1968), Can Heironymus Merkin Ever Forget Mercy Humppe and Find True Happiness? (1969), Tiffany Jones (1973), and the Hammer film Hands of the Ripper (1971) as an ill-fated prostitute. She also appeared in the Barbra Streisand film Yentl (1983), Carry On Columbus (1992), Colour Me Kubrick (2005) and Scoop (2006) directed by Woody Allen.

Baron's early television roles included small parts in Crossroads (1964), Up Pompeii (1970), Z-Cars (1971), and the British horror film Hands of the Ripper (1971). Baron appeared on television in BBC-3 (1965), a series in the vein of That Was The Week That Was, involving some of the same performers. She also alternated with Annie Ross as the resident singer on Not So Much a Programme, More a Way of Life (1965). Baron has taken part in the BBC science fiction series Doctor Who three times. She was heard as a singer in the 1966 serial The Gunfighters. She appeared in front of the cameras as Captain Wrack in the 1983 serial Enlightenment, and again in 2011 in Closing Time as Val.

In 1966, Baron married John M. Lee. They have two children, Sarah and Morgan.

In the 1970s and 1980s, Baron co-starred in the ITV sitcom Oh No, It's Selwyn Froggitt! and the forgotten BBC sitcom A Roof Over My Head. She had small parts in Minder and Last of the Summer Wine. In 1986, she acted in a party political broadcast for the SDP–Liberal Alliance. Baron also appeared in the 1987 Christmas special of The Two Ronnies. Baron then went on to appear in the BBC Two comedy series KYTV.

Baron is best known for playing Nurse Gladys Emmanuel in the popular BBC sitcom Open All Hours with Ronnie Barker and David Jason which ran for four series in 1976, 1981 to 1982 and in 1985, and was subsequently voted eighth in Britain's Best Sitcom in 2004.

In 1987, Baron starred in the London production of the musical Follies at the Shaftesbury Theatre. In 2007, she starred with Orlando Bloom and Tim Healy in a stage version of In Celebration. In May and June 2009, she appeared at the Menier Chocolate Factory in a production of Rookery Nook by Ben Travers.

In the 1990s, Baron played Auntie Pat in five episodes of the ITV sitcom The Upper Hand (1992–93). Baron then went on to star in the children's television series Come Outside (1993–97) playing Auntie Mabel, an everyday woman living in a bungalow, set in Denham flying round on various adventures in her spotted aeroplane with her dog Pippin.

From 1993 to 1997, she played Auntie Mabel in BBC children's programme Come Outside and in 2006, she starred as Linda Clarke in EastEnders, before returning from 2008 to 2009 and again in 2016.

In 1997, Baron played Renee Turnbull in Coronation Street and took guest roles in Dinnerladies (1998), The Mrs Bradley Mysteries (1998), Sunburn (1999), Nancherrow (1999) and Goodnight Sweetheart (1999).

Baron continued to work regularly on television and the stage in the 2000s, with credits including Fat Friends (2000–2005), The Bill (2000), Doctors (2000, 2006, 2010, 2011 and 2014), Peak Practice (2001), Holby City (2002 and 2006), Down to Earth (2005), Rome (2005) and Casualty (2009).

From October 2010 to February 2011, Baron starred with Maureen Lipman and Roy Hudd in a West End production of When We Are Married by J.B. Priestley. In March 2013, she appeared in a production of D.H. Lawrence's play The Daughter-in-Law at the Sheffield Crucible. She appeared at the Minerva Theatre, Chichester from April to May 2014 in Stevie by Hugh Whitemore, alongside Zoe Wanamaker and Chris Larkin. Baron again appeared in the play from March to April 2015 at the Hampstead Theatre.

In 1995, Baron voiced the character of Nanny Ogg in the BBC Radio 4 adaptation of the Discworld novel Wyrd Sisters. In April 2012, Richard Kates released an album entitled There's Something About You, which featured Baron performing the track "A Hard Man is Good to Find". On 11 May 2012, Baron appeared in the Afternoon Play on BBC Radio 4, Mrs Lowry and Son, playing the mother of Artist LS Lowry.

On 26 December 2013, Baron reprised her role in a special one-off episode of Open All Hours on BBC One, entitled Still Open All Hours. It was watched by 12.23 million viewers, almost a 40% share in audience figures on Boxing Day. The following day, Baron took part in Open All Hours: A Celebration, a programme looking back on the sitcom. Following the success of the Christmas episode, the cast made a full series, which began on 26 December 2014. A second series started in December 2015. A third series began in December 2016, but Baron was unable to return because of other commitments. Baron did not appear in the fourth series. It is believed that she has quit the show.

In December 2016 Baron made a guest appearance in a Christmas special of Citizen Khan and in January 2017 she appeared in an episode of Father Brown.