After the war Ellis decided to study at Trinity College, Cambridge. After graduating in 1920 he did research work at the Cavendish Laboratory, Cambridge. The director there was Sir Ernest Rutherford. Chadwick also worked there. In 1921 Ellis had become a fellow of Trinity College and was made assistant lecturer in natural science. In 1925 he married the daughter of a rich Polish shipbuilder. In 1929 he was elected a fellow of the Royal Society.