Dellow was born in London and educated at William Ellis School, Highgate, and the Royal Grammar School, High Wycombe. After leaving school, he worked for Shell and did his national service in the Royal Army Ordnance Corps, working in personnel selection. In 1951, he joined the City of London Police as a Constable. Rising to Chief Inspector and attending Bramshill Police College, he transferred to Kent County Constabulary as a Superintendent in 1966. He was promoted Chief Superintendent in 1968 and Assistant Chief Constable in 1969. In the same year, he became the first police officer to attend the Joint Services Staff College.
In 1973, he transferred to the Metropolitan Police as Deputy Assistant Commissioner (Traffic Planning). In 1975, he became DAC (Personnel), in 1978 he took over No.2 Area, and in 1979 he became DAC (Operations). In this post, Dellow commanded the police operation in the Iranian Embassy Siege in 1980. Later that year he was appointed DAC (Inspectorate). He was appointed Officer of the Order of the British Empire (OBE) in the 1979 New Year Honours.
On 10 May 1982, he was appointed Assistant Commissioner "B" (Traffic). In 1982, he headed the review of Buckingham Palace security after an intruder, Michael Fagan, managed to get into the Queen's bedroom. In March 1984 he was transferred as Assistant Commissioner "C" (Crime). The reorganisation in 1985 meant he was the last officer to hold the post of Assistant Commissioner "C" and the first to hold the new post of Assistant Commissioner Specialist Operations (ACSO). He was appointed Commander of the Order of the British Empire (CBE) in 1985. In 1987, he was promoted to Deputy Commissioner, holding the post until his retirement in 1991. He was vice-president of the Association of Chief Police Officers (ACPO) from 1988 to 1989 and president from 1989 to 1990. He was knighted in the 1990 Birthday Honours.
- "Twenty soldiers freed captives in raid on Iranian Embassy", The Times, 7 May 1980
- The London Gazette: . 29 December 1978.
- "Siege chief promoted", The Times, 24 February 1982
- The London Gazette: . 11 March 1982.
- "Intruder 'found at Queen's bedside', The Times, 12 July 1982
- "Siege chief to lead London's detectives", The Times, 6 March 1984
- The London Gazette: . 15 June 1990.