George William Ziadie was born in Jamaica in 1949, one of four children of department store owner Michael George Ziadie and Gloria Dey (née Smedmore). At birth, she had a genital malformation (a fused labia and deformed clitoris). Medical advice at the time was to assign her as a male so that she could live what was deemed a normal life. Though her family life was otherwise happy, Campbell has since spoken and written of the many personal issues she faced being raised as a boy when she was physically female.

On 23 March 1974, after having known him for only five days, she married Lord Colin Ivar Campbell, the younger son of the eleventh Duke of Argyll. She has said of him, “He had the strongest personality of anyone I had ever met – he simply exuded strength, decisiveness and charm.” However their relationship quickly soured. The couple split after nine months over the scandal surrounding her birth certificate. The couple divorced after 14 months. She successfully sued several publications that claimed she was born a boy and had subsequently undergone a sex change, and accused her former husband of selling the untrue story for money.

Campbell is best known for her books on Diana, Princess of Wales, and Queen Elizabeth The Queen Mother. Her 1992 book, Diana in Private: The Princess Nobody Knows, provided information about Diana’s struggle with bulimia and her affair with James Hewitt (insights into these matters deriving from “one of [Campbell’s] closest friends was one of [Diana’s] closest friends”). Campbell was dismissed as a fantasist, but some of her claims were later vindicated. Diana in Private appeared on The New York Times Best Seller list in 1992.

In 1993, she adopted two Russian boys, Misha and Dima.

In 2013 she purchased Castle Goring, a Grade I listed country house in Worthing, Sussex.

She appeared in Comedy Nation, a British TV show. In November 2015, Campbell took part in the fifteenth TV series of I’m a Celebrity…Get Me Out of Here!. The following month, she left the programme before its conclusion “on medical grounds”. In a later interview, Campbell said that she felt bullied into leaving the show by Tony Hadley and Duncan Bannatyne.

In 2016, she featured in a documentary entitled Lady C and the Castle, which was broadcast by ITV. The programme charted her journey in converting her dilapidated castle into a wedding venue. In 2017 she appeared at the castle in an episode of Salvage Hunters on Quest. She also appeared on Through the Keyhole, where Keith Lemon toured Castle Goring.

In June 2019, Campbell said that the Me Too movement was good in some ways, but also “prevented men from being men”. In August, Campbell appeared on Celebs Go Dating, shown on E4. In November she appeared on Good Morning Britain to defend Prince Andrew’s associations with deceased sex offender Jeffrey Epstein, who had been convicted of soliciting a minor for prostitution. In the interview, she said that Epstein was not a paedophile and argued that there was a difference between “a minor and a child”.