Mellor was born in Leeds, Yorkshire to a Catholic father and a Jewish mother. She attended Bretton Hall College at the University of Leeds and graduated in 1983. She still lives in Leeds and has been married to her husband Anthony since 1967. She has two daughters: actress Gaynor Faye and television producer Yvonne Francas.

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 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 serial, 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.

Outside of television, 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" appearing on BBC One in April 2010.

Mellor was appointed Officer of the Order of the British Empire (OBE) in the 2009 Birthday Honours.