Sondheimer, knowing no English, began his schooling in England first at Southend and then at Hailey School in Bournemouth. In 1940, having passed Common Entrance, he attended the part of Highgate School remaining in London, where he obtained School Certificate in nine subjects in 1942. A little over a year later he gained entrance to Imperial College, where he studied until the end of the war, coming top of his year in the final examination.