Robert Sadler was born in Newmarket, the son of a noted racehorse trainer. After studying at Sidney Sussex College, Cambridge, he joined the Royal Air Force as a pilot in 1930. Over the next few years, he held various posts in the RAF and also began to paint, attending art schools in London, Winchester and the USA where he encountered abstract expressionism.

In 1955 he moved back to Newmarket, having retired from the RAF to devote himself full time to painting. He was influenced by de Stael, Peter Lanyon, William Scott, Bryan Winter and Adrian Heath. Sadler had a distinguished exhibiting career, and in 1964 moved to Aldeburgh where he continued to paint until his death in 2001.