The origin of Sadler’s adventurous career was a pupil in his prep school who had been brought up in Africa and entertained his fellows with adventure stories. Intrigued, Sadler left school in 1937 to work on a farm in Rhodesia (now Zimbabwe).
When the Second World War broke out in 1939, he joined a Rhodesian Army artillery unit equipped with howitzers from India’s North-West Frontier. Despite not being strong at maths or geometry at school, he took a keen interest in the angles of fire needed to engage distant targets, and was disappointed when his unit converted to an anti-tank role "where you could look through a telescope straight ahead".