Son of Major James Jackson Raley Trethowan (1893-1974), of 6, Ashleigh Court, Surbiton, Surrey, formerly of High Wycombe, Buckinghamshire,[1] MBE, a retired officer of the York and Lancaster Regiment who worked in business, army welfare, and sports writing for the Sunday papers (including the Sunday Times, Evening Standard, and Yorkshire Post),[2] after leaving the Army, and his first wife, Winifred, née Timms, Trethowan was educated at the independent Christ's Hospital school near Horsham in West Sussex.
He moved to the BBC in about 1963, and was part of Grace Wyndham Goldie's group of heavy hitting journalists which included Richard Dimbleby and Robin Day.
A Conservative, and a close friend of the former prime minister Sir Edward Heath, Trethowan has been criticised for his support of the Security Service "vetting" of BBC employees which has often been seen as a means of weeding out leftists in the corporation.
Trethowan allowed MI5 to remove half the content from a Panorama documentary made by BBC journalist Tom Mangold, this emerged in December 2011, when 30-year-old British government papers were released.
Cautious and conservative-minded, he was responsible for the sacking of Kenny Everett from Radio 1 in 1970 for making a joke suggesting that the wife of John Peyton, the transport minister in the Conservative government, had only passed her driving test because she had "slipped the examiner a fiver".