As a Writer, she began working for Granada Television in the 1980s, writing for their hugely popular soap opera Coronation Street, the most watched programme on the ITV network. In 1989 Mellor also wrote seven episodes for the popular Channel 4 soap Brookside.

She attended Bretton Hall College (now part of the University of Leeds) and graduated in 1983. She still resides in Leeds with her husband Anthony Mellor, who she married in 1967. The couple have two daughters: Actress Gaynor Faye and television Producer Yvonne Francas.

She also wrote for the anthology drama series Dramarama, before in 1988 co-creating the long-running children's drama Children's Ward with her Coronation Street colleague Paul Abbott. She also created the soap opera Families, which aired from 1990 until 1993. Since then she has written a host of highly acclaimed and popular television drama serials, including Band of Gold (1995), Playing the Field (1998) for BBC One, Fat Friends (2000), Between the Sheets (2003) and Strictly Confidential (2006) for ITV.

In 1999 she both wrote and directed the feature film Fanny and Elvis, starring Ray Winstone. In her parallel career as a television Actress, Mellor has appeared in her own adaptation of Jane Eyre (1997) and in other series such as the comedy drama Stan the Man (2002) and in Gifted (2003). In July 2006 another of her dramas aired on BBC One, called The Chase. She both wrote and directed the two-part drama A Passionate Woman, which was based on her 1992 stage play, on BBC One in April 2010. In 2012 she wrote another BBC drama, The Syndicate. This returned for a second series in 2013 and a third series in 2015. In 2014, her series In the Club was first broadcast.

She was the subject of This Is Your Life in 2000 when she was surprised by Michael Aspel. She also appeared on the BBC's Desert Island Discs in 2017.

Mellor was appointed Officer of the Order of the British Empire (OBE) in the 2009 Birthday Honours. In 2016 Mellor was made a Fellow of the Royal Television Society.