Carter-Ruck's first major case was defending the Bolton Evening Post successfully against a libel action brought by the Labour MP, Bessie Braddock, who, they had claimed, had danced a jig in Parliament. He worked for many years at Oswald Hickson Collier & Co and set up his own firm in 1981 when it was suggested that he should retire.
He was known for several high-profile cases against British media outlets such as Private Eye, the BBC and Channel 4. His clients included Princess Elizabeth of Toro, Jani Allan, Randolph Churchill and Sir James Goldsmith , plus many Conservative and Labour politicians including Neil Hamilton, Cecil Parkinson, Norman Lamont, Robert Maxwell and Harold Wilson. He also represented Laurence Olivier, Spike Milligan, Cary Grant and Ranulph Fiennes.
He did legal aid work, and said it was wrong for successful firms to decline it. He was frequently referred to by Private Eye, a regular target of his libel cases, as "Peter Carter-Fuck". Despite being ousted from the firm bearing his name, he continued to act on many high-profile cases during the 1990s. In 1989, he criticised the £600,000 pay-out to Sonia Sutcliffe, who sued Private Eye for libel as excessive. The amount was later reduced to £60,000 on appeal.
At one time, he was the oldest practising solicitor in the country. He finally retired in 1998.
Victories could also be pyrrhic, as in the case of William Roache whom Carter-Ruck encouraged to sue The Sun for saying he was boring. Roache was awarded £50,000 but as the paper had already paid that amount into court, Roache ended up having to pay his own costs and those of The Sun. He tried to sue Carter-Ruck but was unsuccessful and declared bankruptcy.
Despite working for the Law Society between 1971 to 1984, Carter-Ruck was not above disreputable practices. His partner in the firm, David Hooper recalled a case where Heinemann had asked him to check Robert Lacey's biography of the Ford family. He found Carter-Ruck's secretary photocopying the book to submit to the Ford family, whom Carter-Ruck was advising to sue because of libels in the book. Hooper resigned in protest.
Carter-Ruck and his parter Andrew Stephenson were also investigated by the Office for the Supervision of Solicitors in 1998 after a complaint from one of their clients, Chequepoint, which had been engaged in lengthy litigation with a rival firm of bureaux de change, Maccorp, and then discovered that Carter-Ruck and Stephenson were also acting for Maccorp. They were both found guilty of a flagrant conflict of interest.
The late Michael Wheeler QC also delivered a damning verdict on Carter-Ruck after he arbitrated a dispute between Carter-Ruck and his partners at Oswald Hickson during which Carter-Ruck attempted to blackmail his colleague to falsify his testimony. Wheeler wrote "it seems to be... to be a gross interference with the course of justice such as would (if this had been a court case and not simply an arbitration) have constituted a blatant contempt of court. Certainly, if a member of the bar had behaved as Carter-Ruck has done, I should have no hesitation in reporting his conduct to the Bar Council."
Carter-Ruck's former firm remains active in high-profile defamation cases. Not all clients are satisfied, one former client Adam King who won a libel case against the Telegraph opened several web sites against the firm.