Born in Newmarket, Suffolk, the son of a racehorse trainer, Robert Sadler showed great artistic promise as a child, drawing and painting landscapes, portraits, and aeroplanes. After studying Mechanical Engineering at Sidney Sussex College, Cambridge and learning to fly in the University's Air Squadron, in 1930 Sadler began an illustrious career in the Royal Air Force.
Sadler continued to take opportunities to develop his artistic talents while posted in London, Turkey, Denmark, and the USA, attending Heatherly's Art School in London and taking his watercolours and easel out into the Turkish countryside. His affinity for plein air painting continued while studying in art schools in Copenhagen and Winchester; also a skilled portraitist, his talents proved useful as a vice-Chairman of the RAF Officer Selection Board, producing small portrait studies to keep track of different applying officers. After his final posting to Washington DC as part of NATO's intelligence committee, Sadler retired in 1954.
Exposed to the work of the American Abstract Expressionists, artists such as Jackson Pollock, as well as the abstract landscapes of French-Russian painter Nicolas de Staël proved a major influence as Sadler, working professionally from 1955 to his death in 2001, decided to "devote the rest of my life to painting".
In addition to Studio Exhibitions yearly from 1965 to his death in 2001, Sadler's works were exhibited widely throughout his career, in London at The Royal Institute of Painters and Watercolours (1955), The Royal Institute Galleries Summer Salon (1955), and The London Group (1962), as well as the John Moores Liverpool Exhibition (1957-58), Bradford City Art Gallery (1959), CEMA in Belfast (1962) and Phillip Francis Gallery, Sheffield (1984). He also found international success, his work making its way to Washington DC, North Carolina, and even being displayed in the EU Parliament in Strasbourg. More recently, twelve of Sadler's paintings were used to decorate the set for the 2007 Cornwall-based TV series Echo Beach.