Wainwright was born in 1963 in Huddersfield, West Yorkshire, to Harry Wainwright and Dorothy Wainwright (née Crowther). Wainwright was brought up in Sowerby Bridge where she attended Triangle C of E Primary School and Sowerby Bridge High School. She attended the University of York, reading English and Related Literature. She has an older sister, Diane Hilton, a housing professional with The Guinness Partnership.

Wainwright said that she had always wanted to write, and had started writing when she was young, from the time she was 9 years old, and wanted to write for Coronation Street. She said that when she was 16 years old in 1980 she saw a play called Bastard Angel by playwright Barrie Keeffe at the Royal Shakespeare Company and was deeply interested in the short sentences and naturalistic approach to dialogue.

Wainwright, who lives in Oxfordshire, married Ralph “Austin” Sherlaw-Johnson, an antiquarian sheet music dealer, in 1990. They have two sons and own two Maine Coon cats.

Wainwright based the plot of her series Last Tango in Halifax on the story of her mother who was widowed in 2001. Her mother, Dorothy, moved to Oxfordshire to live with her daughter and rediscovered a lost love via Friends Reunited. With her mother’s permission, Wainwright developed the story of how she remarried so rapidly, showing extracts from the series to her mother before broadcast.

While at the University of York, Wainwright took an original play called Hanging On to the Edinburgh Festival and found an agent, Meg Davis, for her writing in the process. Meanwhile, she worked as a bus driver. When she was 24, she left the driving job after she started writing for the Radio 4 series The Archers. One of her contributions was to write an atypical story for the long-running radio soap in which the village shop was robbed. After that she wrote for Coronation Street, developing her writing skills, from 1994 to 1999. She has since said that working on continuing drama was “a great education in discipline and a lesson that great stories are hard work”. She was mentored by Kay Mellor who encouraged her to stop writing for soaps and to concentrate instead on original work. She created the TV series At Home with the Braithwaites about a woman who had secretly won the lottery. The programme was nominated for multiple awards. In 2006 she wrote the drama series Jane Hall, which depicts the life of a female bus driver in London. Wainwright drew on her own experiences in scripting the series.

She won the 2009 Writer of the Year Award given by the RTS in 2009 for Unforgiven which took several awards including best TV series.

In 2011 she wrote Scott & Bailey, a series about two female police officers. The idea for the series came from the actresses Suranne Jones and Sally Lindsay and former Detective Inspector Diane Taylor, who assisted with bringing the series to air.

When she told the story to Nicola Shindler, she suggested she turn her mother’s experience into a television series. Shindler became the series’ executive producer. Both Last Tango in Halifax and her crime series Scott & Bailey were turned down by both the BBC and ITV before being accepted retrospectively. The former was voted by BAFTA to be best series in 2012 and Wainwright was given the award for best writer.

In 2016, Wainwright was made a Fellow of the Royal Television Society.

Wainwright wrote and directed a two-hour drama special for BBC One entitled To Walk Invisible: The Bronte Sisters, which aired on BBC One in 2016 and in the US in 2017. Its subject is the Brontë family, particularly the relationship the three sisters, Anne, Emily and Charlotte, had with their brother, Branwell. While working on the drama, Wainwright said “I am thrilled beyond measure that I’ve been asked by the BBC to bring to life these three fascinating, talented, ingenious Yorkshire women.”

In 2019, Wainwright’s Gentleman Jack, a drama about the 18th-century Yorkshire landowner, diarist, and open lesbian, Anne Lister, played by Suranne Jones, and Lister’s courtship of Ann Walker, played by Sophie Rundle, premiered on both BBC One in the UK and HBO in the US.

Wainwright was appointed Officer of the Order of the British Empire (OBE) in the 2020 Birthday Honours for services to writing and television.