Lang was born in Marylebone, London in 1953, daughter of actors Jeremy Hawk (1918–2002) and Joan Heal (1922–1998). In 1988 she married actor Hugh Fraser, with whom she has one daughter, Lily.

After a small part as 'Girl in Bath' in Play for Today in 1980 she appeared in the 1980 miniseries To Serve Them All My Days, following this she had several one off appearances and small parts until she took the role of 'Kate' in both series of the sitcom Dear John (1986–87). Following Dear John she played Martha in The Bretts (1987–88) during which she acted alongside her Future husband, Hugh Fraser and during the same period also appeared as Sheila Walsh in Bust. Lang also had a lead part in the television series Inspector Alleyn Mysteries in which she played the Artist Agatha Troy between 1990 and 1994.

In 1988, Lang appeared in the BBC Radio 4 play Hard of Hearing opposite Graham Blockey. She took part in Stilgoe's Around, also broadcast on Radio 4 and BBC Radio 2. Second Thoughts was based on a radio programme originally broadcast on Radio 4.

Lang is perhaps best known for her starring roles in two 1990s sitcoms – as Bill Porter, alongside Gary Olsen as husband Ben, in the BBC TV comedy 2point4 Children and as Liza Ferrari in Second Thoughts.

In 1991, she took a lead role in 2point4 Children alongside Gary Olsen. The sitcom would run for 8 series until it ended in 1999 due to the illness of Olsen leading to his death in 2000. In 2000, she appeared as Christine Hamilton in the television film Justice in Wonderland.

Following 2point4 Children, Lang has rarely appeared on television, instead taking on more theatre work. She has, however, made guest appearances in several programmes including; playing the ill-fated Business woman, Elspeth Inkpen-Thomas in Midsomer Murders, alongside John Nettles and television presenter Quinnie Dorrell in "The Gongoozlers", a 2004 episode of the murder mystery series Rosemary and Thyme, and a couple of episodes of the BBC daytime soap opera Doctors. In 2009, she made a brief return to sitcom, playing Margot in an episode of BBC sitcom My Family, and then in 2014 playing Marina Fairchild in Citizen Khan, another BBC series.

She is now a Vice-President of the theatre charity – The Theatrical Guild, having been involved with them for many years. Belinda was chairman from 2009 to 2011 and continues to help promote them amongst the theatrical profession.

Lang appeared with Nicholas Farrell and David Robb in Alan Bennett's "Single Spies" play during 2016. Her other theatre credits include Present Laughter (1981), The Clandestine Marriage (1984), Dead Funny (1995), Blithe Spirit (1997), and Hay Fever (2006). BBC Proms 2017 "Aunt Eller" in Oklahoma with the John Wilson Orchestra (2017).