Sims was born in 1930, the only child of John Henry Sims (1888-1964), station master of Laindon railway station in Laindon, Essex, and his wife Gladys Marie Sims, née Ladbrook (1896-1981). Sims's early interest in being an actress came from living at the railway station. She would often put on performances for waiting passengers. She decided that she wanted to pursue show business during her teens, and soon became a familiar face in a growing number of amateur productions locally. One of her first stage appearances was as Miranda Bute in the Langdon Players production of Ester McCracken's comedy 'Quiet Wedding' in May 1946.
In 1946, Sims first applied to RADA, but her audition was unsuccessful. Her first audition included a rendition of Winnie the Pooh. She did succeed in being admitted to PARADA, the academy's preparatory school, and finally, on her fourth attempt, she graduated and was trained at RADA. She graduated from RADA in 1950 at the age of 19. One of her first stage performances was in the 1951 pantomime, The Happy Ha'penny, opposite Stanley Baxter at Glasgow's Citizens Theatre.
Sims appeared in a number of Brian Rix's Aldwych Theatre farces including the well known "I say! Nawks!", but revue was Sims's greatest medium, especially in the works of Peter Myers. In 1958, she got a part in Peter Coke's play Breath of Spring, which opened at the Cambridge Theatre in March, transferring to the Duke of York's Theatre in August 1958, which ran until April 1959. Sims preferred film to stage work. "It was, of course, lovely to be in a successful play, to have the excitement of performing a hit to packed houses (and, not least, the assurance of a regular income for the foreseeable future). But, on the other hand, I found it extremely difficult to keep a performance fresh, and I'd soon get bored."
Sims made her first film appearance in Will Any Gentleman? with George Cole in 1953, closely followed by Trouble in Store with Norman Wisdom. In 1954, she appeared as Miss Dawn in The Belles of St Trinian's, and made a cameo appearance in Doctor in the House, opposite Dirk Bogarde as the sexually repressed Nurse Rigor Mortis. Sims became a regular in the Doctors series, which was produced by Betty E. Box, and was hence spotted by Box's husband Peter Rogers.
She had a small part in the 1957 film Carry on Admiral, unrelated to the later Carry On series and with no other cast members in common with the series.
In 1958, Sims received a script from Peter Rogers; it was for Carry On Nurse. The film Carry On Sergeant had been a huge success at the box office and in the autumn of that year, Rogers and director Gerald Thomas began planning a follow-up.
She first starred in Carry On Nurse, then Carry On Teacher, followed by Carry On Constable and Carry On Regardless, and this sealed her future as a regular Carry On performer. Following a bout of ill health, Dilys Laye had to be brought in to take her place in Carry On Cruising at very short notice; however, Sims rejoined the team with Carry On Cleo. Her role in this was to set the tone for the rest of the Carry On films. "...[F]or once the costumes were made for me, rather than my having to resort to some old dress that had been used before and had to have a new panel sewn in the back to accommodate my girth – which by now was unvaryingly plump."
Sims' characters evolved from objects of desire in the early films to frumpy, nagging wives in the later ones, epitomised by the Emily Bung role in Carry On Screaming. Following the success of Carry On Cleo, she stayed with the films all the way though to the final one in the original series, Carry On Emmannuelle. Sims appeared in 24 Carry On films in all; she did not return for the one-off revival film, Carry On Columbus (1992). However, she did appear alongside Kenneth Williams in the radio show Stop Messing About in 1969–70. Prior to this she had worked on the BBC Radio comedy Play it Cool, with Hugh Paddick and Ian Carmichael.
After the Carry On series ended in 1978, Sims continued to work on television. She appeared opposite Katharine Hepburn and Laurence Olivier in the award-winning 1975 television film Love Among the Ruins and had a recurring role as Gran in the BBC comedy series Till Death Us Do Part. From 1979 until 1981, she played the recurring character Mrs Bloomsbury-Barton in Worzel Gummidge for Southern Television. During 1986 and 1987, Sims starred as Annie Begley alongside Angela Thorne in the Yorkshire Television sitcom Farrington of the F.O. Also in 1986, Sims appeared in the long-running BBC science fiction series Doctor Who in the four episodes of The Trial of a Time Lord: The Mysterious Planet as Katryca. She also played Miss Murgatroyd in the Miss Marple adaptation A Murder is Announced, Betsy Prig in a star-studded adaptation of Dickens' Martin Chuzzlewit and Lady Fox-Custard in Simon and the Witch. In 1987 she joined the cast on And There's More and was paired up with Nicholas Smith for a number of sketches for each episode as an old couple.
In 1989, she appeared as a medium in the video for Morrissey's "Ouija Board, Ouija Board".
She played Mrs Wembley in the BBC comedy series On the Up, which starred Dennis Waterman and ran from 1990 to 1992. From 1994, she played Madge Hardcastle, drum playing wife to Rocky Hardcastle played by Frank Middlemass, and stepmother of Geoffrey Palmer's character Lionel in As Time Goes By. Sims also appeared in episodes of the hit television comedy series Only Fools and Horses, the 1987 Christmas Day special The Frog's Legacy, and The Goodies, in the One Foot in the Grave special One Foot in the Algarve, and made a guest appearance in a sketch show with Victoria Wood.
In her later years, Sims fought a long battle against depression. This was worsened by the deaths of her agent Peter Eade, her best friend Hattie Jacques and her mother, all within a two-year period, after which she fell into alcoholism. Sims suffered from Bell's palsy in 1999 and fractured her hip in 2000, but recovered well. However, her alcoholism was beginning to dominate life in her rented Kensington flat, and she described herself as "the queen of puddings." After assessment by a doctor, she was offered a place in a rehabilitation centre, but declined. Offered the opportunity to write her autobiography, she took a role in the BBC television film The Last of the Blonde Bombshells, alongside her As Time Goes By castmate Dame Judi Dench and Olympia Dukakis.
During 1963, Sims made several recordings. "Hurry Up Gran" / "Oh Not Again Ken" was issued as a single, followed by "Spring Song" / "Men". Both were produced by George Martin for the Parlophone label but neither single made an impact on the UK Singles Chart. This did not deter her from releasing a third and final single during 1967, "Sweet Lovely Whatsisname" / "The Lass With the Delicate Hair". Again it failed to chart, and as a result the singles are now quite rare. As of 2009, both "Spring Song" and "Men" are available for the first time through iTunes and other download services, as well as on CD as part of re-issues of the comedy compilation albums Oh! What a Carry On! and Laugh A Minute. Sims also featured on an original cast recording of The Lord Chamberlain Regrets in 1961, as well as The Water Gypsies.
Sims, like her fellow Carry On star Kenneth Williams, never married. Williams, who was homosexual, did however propose a marriage of convenience to her, which she promptly declined. From 1958, she lived for three years with fellow actor Tony Baird but, every time her parents visited, she asked Baird to remove all of his belongings from their London flat.
After she told her mother on a visit that she was living with Baird, her father wrote her a stern letter, condemning the relationship. Sims replied, telling her parents that they had to come to terms with Tony being an extremely important part of her life. For the next six months she had no contact with her parents. Sims was a devoted daughter and found the separation from her parents difficult.
The relationship with Baird began to flounder, Sims said, due to her success and Tony Baird's failure as an actor. Sims writes "Had house husbands been in vogue in those days we'd have made an excellent couple, since Tony was not very successful as an actor and I soon became the main breadwinner. If we had been able to accept that I would go out and earn the money and he would concentrate on running the home, things might have turned out better.... For three years I was besotted with this loveable reprobate, but then the icing on the cake began to chip off and the love started to wear thin. I was virtually keeping him and the friction of the situation was getting harder to bear." Of the break-up, which was finally triggered by Sims returning from a tour to find Baird had not done any washing or housework, she wrote "I could tell that he was genuinely heartbroken, and so was I, but I had to do it for my own survival."
Following this came a relationship with John Walters whom Sims had known for a long time. He had been assistant stage manager for the play High Spirits, in which she appeared. They had had an 'innocent' romance at the time, but they embarked on a more serious relationship after Sims's break-up with Baird. However, Sims never felt it would be a long-term relationship: John was a much moodier character than Tony. During what she described as the 'one broody phase of my life', they discussed marriage and children, but it came to nothing and the relationship, the last serious one of Sims's life, ended after around two years of living together.
"I never married because the right person never came along... I leave others to seek for darker explanations. For me it's extremely simple!"
The tone of Sims's autobiography High Spirits is revealing (though not sensationalist), frank and sometimes mordant:
"In Doctor at Sea I was cast again as the Plain Jane character ... my rival in love was played by ... Brigitte Bardot. Joan Sims versus Brigitte Bardot. I'll leave you to guess which of us got her man."
"Then the effects of the tablets rapidly started wearing off – as is the way with Benzedrine – and suddenly I was feeling worse than I had ever felt in my life."
"I learned the hard way how deflating it can be to get too excited by a prospect before you know for sure that it will come off. The worst aspect of this fiasco was that I was now not only jobless but homeless."
"I was always useless at flirting, and simply did not know what needed to be done in order to snare my target... I always ended up resorting to jokes, and most men don't like funny women. They like to do the jokes."
"I've never been able to understand women who have this burning desire to have children. I've never had those feelings in any depth."
High Spirits concluded with Sims in reflective and rueful mood. Having been disappointed to miss out on the part in a BBC adaptation of Vanity Fair, she is somewhat crestfallen to discover that there are only two entries on her 'Trivia' page on IMDb.com. It ends:
"The last couple of years have seen more lows than highs .... my long-held view that whether you're up or you're down, there's only one way to react to whatever life throws at you. Carry on."
Sims entered hospital in November 2000, and complications of a routine operation caused her to slip into a coma. Her lifelong friend and stand-in Norah Holland spoke of the doctors' amazement at her strength and courage throughout her final illness.
On 27 June 2001, ten minutes before she died, Norah Holland spoke to her gently about Kenneth Williams, Hattie Jacques and their time on the Carry On films. Sims died from liver failure and diverticulitis, with diabetes and COPD cited as contributory factors. She was cremated at Putney Vale Crematorium, and her ashes scattered in the grounds there.