Blackman was born in West Ham, London to a statistician father. She trained as an actress at the Guildhall School of Music and Drama in London, after persuading her father that an appropriate birthday gift would be acting lessons.
Blackman's films include: Quartet and So Long at the Fair with Dirk Bogarde, Life at the Top with Laurence Harvey, The Virgin and the Gypsy, Shalako with Sean Connery and Brigitte Bardot and Something Big with Dean Martin. She played the role of Hera in Jason and the Argonauts (1963). She also did an overdub for an actress in the same film providing the voice for the character of Medea. More recently, she had small roles in the films Bridget Jones Diary and Hot Gold.
She spent most of 1987 at the Fortune Theatre. From 2005 to 2006 she toured the country as Mrs Higgins in My Fair Lady. Her show Word Of Honor, premiered in October 2006. In April 2007 she took over the role of Fraulein Schneider from Olivier Award-winning actress Sheila Hancock, in Cabaret at the Lyric Theatre in London's West End. She left the show at the end of September 2007.
In a 1965 episode of The Avengers, titled "Too Many Christmas Trees", John Steed received his Christmas cards, one of which was from Cathy. "A card from Mrs Gale!", Steed exclaims in delight. Then, reading the inscription, he says, in a puzzled voice, "Whatever can she be doing at Fort Knox...?". It was an inside joke, as Blackman was filming Goldfinger at the time.
In 1986, she played the role of Professor Lasky in the Doctor Who serial The Trial of a Time Lord. From 1990 to 1995 she appeared as Laura West on The Upper Hand. Blackman took a guest role on Midsomer Murders as ex-racing driver Isobel Hewitt in the episode A Talent for Life. She also, in September 2004, briefly joined the Coronation Street cast in a storyline about wife swapping.
In 2007, she participated in the BBC TV project The Verdict, as one of 12 well known figures forming a jury to hear a fictional rape case. The series was designed to explore the jury system. She was sworn in as a juror under the name 'Honor Kaufmann'.
Blackman was featured, alongside Chris Tarrant, in the show Lose A Million.
A song she recorded with Patrick Macnee during 1964, "Kinky Boots", was a surprise hit, peaking at #5, in 1990 after it was played incessantly by BBC Radio 1 breakfast show presenter Simon Mayo. After her appearance in Goldfinger, Blackman recorded a full album of songs entitled Everything I've Got.
She married twice: Bill Sankey (1946 – 1956) and the British actor Maurice Kaufmann (1963 – 1975), with whom she appeared in the film Fright (1971); they adopted two children, Barnaby and Lottie.
She is a signed supporter of Republic, The Campaign for an Elected Head of State, the UK campaign to replace the monarchy with a republic. She declined a CBE honour in 2002. She has also been a prominent supporter of the Liberal Democrats.