Porter was born in Napier, New Zealand. Her first professional work was touring with the New Zealand Players Trust. She was acclaimed for such roles as Jessica in The Merchant of Venice and Juliet in Romanoff and Juliet. She also performed in revues and musicals, including Love from Judy and The Solid Gold Cadillac.

Porter moved to Britain in 1958 after winning a Miss Cinema talent competition for young actresses organised by Rank, with the prize of a round-the-world trip and a film test in London. Although the test was probably little more than a publicity stunt, she decided to stay and was soon acting in the theatre. Look Who's Here at the Fortune Theatre in Drury Lane was her first West End appearance.

She followed this with the role of Connie in Neil Simon's first West End play, Come Blow Your Horn, and a string of other appearances including those in The Duel, The Dragon Variation, Murder in Mind, Anastasia and Deadly Nightcap. Porter also had two roles in Stephen Sondheim's Sunday in the Park with George, at the National Theatre in 1990 and played Olivia in Twelfth Night at the Shaw Theatre, and Rosalind in As You Like It at the Ludlow Festival. She also toured in Australia, in Jeffrey Archer's Beyond Reasonable Doubt and later, in The King and I.

Her film appearances include The Cracksman, Two Left Feet, Live Now - Pay Later, Jane Eyre, Perchance to Dream and The Martian Chronicles among others. She appeared in several television productions and is probably best remembered for her roles in The Forsyte Saga and The Protectors. She had already appeared in Madame Bovary and Judy Paris, when in 1967, she was cast as Irene in the BBC's 26-part serial The Forsyte Saga. The story of wicked Soames Forsyte's marriage - including the harm to his wife - gripped the attention of British viewers and later audiences around the world. Porter became an overnight star as the tragic Victorian woman trapped in a loveless marriage and won a BAFTA award for her performance. Later, she did a stage tour of the programme, which had won her so many admirers.

In 1970, she was appointed an Officer of the Order of the British Empire (OBE).

Riding high on her success, she starred in the 1968 comedy series Never a Cross Word and three years later, alongside actor Robert Vaughn in puppet master Gerry Anderson's live-action series The Protectors. She appeared in Doctor in Charge, Anne of Green Gables and the BBC serial David Copperfield as the mother of Copperfield's best friend - James Steerforth.

Porter played the title role in the 26-part daytime serial For Maddie With Love, as a woman with only a few months left to live. Her screen husband was played by Ian Hendry. The programme ran for two series, in 1980 and 1981. She also appeared as a dotty games mistress in a Dick Emery sketch and was the subject of This is Your Life.

She also starred in an early episode of The Avengers called "Death on The Slipway".

Her first husband, Bryan O'Leary, died in 1970 after an accidental drug overdose. During their 13-year marriage she miscarried three times. In 1975, the actress married actor Robin Halstead, 11 years her junior, after giving birth to a daughter, Natalie; the couple divorced in 1987.

She died in Wandsworth, London, in 2001 from leukemia. She was cremated at Putney Vale Crematorium and her ashes buried in the cemetery there.