After the war, he ran a fruit farm in Sussex. His wife Victoire Evelyn Patricia "Paddy" Bennett, whom he married in 1942, was then secretary to the Writer Ian Fleming. She is reported to have been a model for the character Miss Moneypenny, secretary to James Bond. She was her husband's secretary and chairman of the Conservative MPs' Wives, and was awarded the DBE in 1991.
At the 1951 snap general election, Ridsdale stood as the Conservative Party candidate in the London seat of Paddington North, but lost to the sitting Labour MP william Field.
In 1954, the National Liberal MP for Harwich, Sir Stanley Holmes was elevated to the peerage as Baron Dovercourt, and Ridsdale was selected as 'Conservative and Liberal' candidate to contest the consequent by-election. He was elected on 11 February 1954, defeating Labour's Miss Shirley Catlin (later Shirley Williams, fighting her first election), and he served for nearly forty years, being re-elected at nine subsequent general elections: 1955, 1959, 1964, 1966, 1970, February 1974, October 1974, 1979, 1983 and 1987. Ridsdale stood down at the 1992 general election, and was succeeded by the Conservative Iain Sproat.
After supporting Prime Minister Anthony Eden during the 1956 invasion of Suez, Ridsdale served from 1957-58 as the Parliamentary Private Secretary (PPS) to John Profumo, the Parliamentary Under-Secretary of State for the Colonies. From 1958-60 he was PPS to the Minister of State for Foreign Affairs. His ministerial career was brief, as Parliamentary Under-Secretary of State for Air from 1962-64.
Retaining his wartime interest in Japan, Ridsdale concentrated on improving Anglo-Japanese relations and developing trade links. He was Chairman of the British Japanese Parliamentary Group from 1964–92 and the leader of successive Parliamentary delegations to Japan. He was also Member of the North Atlantic Assembly from 1979-92.
Returning to the backbenches, he continued to mark himself as traditional rightwing Conservative, opposing tax increases and supporting capital punishment. In 1968, he supported Enoch Powell after Powell's controversial anti-immigration "Rivers of Blood speech", calling him "the Winston Churchill of today".
He received the CBE in 1977 and was knighted in 1981.
