He was born in Northampton and studied at the Royal College of Music before joining the London Philharmonic Orchestra as a trumpeter (he eventually became principal trumpet). By the end of the 1940s he was concentrating entirely on composition. He was made a CBE in 1970, and was knighted in 1993.
Arnold is a relatively conservative composer of tonal works, but a prolific and popular one. He has acknowledged Hector Berlioz as an influence, and several commentators have drawn a comparison with Jean Sibelius. Arnold's most significant works are generally considered to be his nine symphonies. He has also written a number of concertos, including one for guitar for Julian Bream, and one for harmonica for Larry Adler. His sets of dances (two sets of English Dances, and one set each of Scottish Dances, Irish Dances, Welsh Dances and Cornish Dances) are in a lighter vein and also popular (one of the English Dances was used as the theme music for the British television programme What the Papers Say).
Arnold has also written a number of film scores, winning an Academy Award for The Bridge on the River Kwai (1957), and also providing music for The Inn of the Sixth Happiness (1958) and Whistle Down the Wind (1961) among others. He conducted the London Philharmonic Orchestra in the recording of Deep Purple's Concerto for Group and Orchestra, and conducted the London Symphony Orchestra in the group's organist Jon Lord's Gemini Suite.