Baynes was born in Boscombe Hospital, Bournemouth. Her mother was Margot Findlay and her father was Leslie Baynes, an English aeronautical Engineer, who designed what is believed to be the oldest flying glider in the United Kingdom. As a girl in the 1970s Baynes attended the Elmhurst Ballet School in Camberley in Surrey, where a contemporary was the Actress Laura Hartong.

Her stage career has involved many leading roles including in 1979 in John Osborne’s Inadmissible Evidence at the Royal Court Theatre, in 1984 a comic performance alongside Maureen Lipman and Lionel Jeffries in the Theatre of Comedy’s See How They Run; in 1991 she appeared with Edward Fox in The Philanthropist at Wyndham's Theatre and in 1997 as Lady Fidget in william Wycherley’s The Country Wife . Her most recent stage performance was in 2004 as Shirley in Revelations by Stephen Lowe at the Hampstead Theatre.

Baynes has also appeared extensively on television including in 1981 with Sir John Gielgud in Agatha Christie’s The Seven Dials Mystery, in 1985 with Pauline Collins and Michael Gambon in The Tropical Moon Over Dorking, in 1990 as the wife of Stephen Fry in Simon Gray’s Old Flames, in 1993 as Hilda in Ken Russell’s Lady Chatterley’s Lover and with Glenda Jackson in The Secret Life of Sir Arnold Bax. She appeared as Vera Rowley in the BBC series The Hour in 2011 and was also to be seen in BBC1's The Casual Vacancy in 2015.

During her career she has received three best Actress nominations for her performances; in 1991 as Rita in Henrik Ibsen’s Little Eyolf (Off-West End Awards), in 1992 as Maddy in Michael Wall’s Women Laughing (Manchester Evening News Awards) and as Marilyn Monroe in Marilyn Bowering’s Anyone Can See I Love You (Sony and Prix Italia Awards).

She began her acting career at just 17 as an acting ASM in repertory theatre. She was married to film Director Ken Russell from 1992 to 1999; they had one son, Rex. She has also been credited as Hettie Baynes, Henrieta Baynes, Henrietta Baynes, Hetty Russell and Hetty Baynes Russell. She obtained a BA in Philosophy from Birkbeck College London in 1990.

In 2008 Baynes unsuccessfully sought financial provision as a "dependant" against the estate of the late Mary Spencer Watson, daughter of the Artist George Spencer Watson, including the family home in Purbeck, Dorset, where Baynes had spent most of her formative years.