On 31 December 1933, Margetson retired from the Army, transferring to the Regular Army Reserve of Officers.[11][12] He joined the Metropolitan Police, entering directly as a chief inspector and taking the position of senior administrative officer at No.4 District (South London) headquarters on 1 December 1933. He was later promoted to Superintendent and took command of "R" Division (Blackheath). In August 1936 he was promoted to Chief Constable and became deputy commander of No.2 District (North London). In February 1938 he was transferred to the same post in No.1 District (West Central London), and in October 1938 to the same post in No.3 District (East London). In February 1940 he was promoted to Deputy Assistant Commissioner at "A" Department (Operations and Administration) of Scotland Yard and in August 1940 he was given command of No.1 District. In March 1946 he received the new rank of commander.
In June 1946, he was promoted to Assistant Commissioner "D" (Personnel and Training) and in October he transferred to become Assistant Commissioner "A" (Operations and Administration). In 1950, he applied for the vacant office of Commissioner of the City of London Police. The job went to one of his colleagues, Arthur Young, who had succeeded him as Assistant Commissioner "D".[citation needed]
He retired on 2 January 1957 (his 63rd birthday) and joined the board of Securicor, serving as chairman from 1960 to 1973, when he became honorary president. He was instrumental in the disarming of cash in transitsecurity guards in 1964, having always disliked the idea of private guards carrying firearms.
In 1918, Margetson married Diana Thornycroft, daughter of Sir John Thornycroft. They had two sons; the elder was killed in action in 1943.
The National Portrait Gallery holds two 1957 photographic portraits of Margetson by Elliott & Fry.