Birley was educated at Rugby and Balliol. He began his career as a history master at Eton in 1926 and in 1935 was appointed as headmaster of Charterhouse. During this time, he was the principal author of the Fleming Report of 1944 on the relationship between the public schools and mainstream education. In 1947, after the Second World War, he became Educational Advisor to the Control Commission in the British Zone in Germany, responsible for educational reconstruction, and played an important role in the rewriting of Nazi history textbooks, removing their racist bent. On his return to the UK in 1949 he was appointed headmaster of Eton, where he remained until 1963. Also in 1949, he was invited by the BBC to deliver the annual Reith Lectures. In these radio broadcasts, titled Britain in Europe: Reflections on the Development of a European Society, Birley considered the history and future impact of Britain's increasing involvement with Europe. He subsequently became a visiting Professor at Witwatersrand University, South Africa from 1964-1967, and was Professor and Head of Department of Social Science and Humanities at City University from 1967-1971. He wrote and lectured extensively on education, apartheid and human rights issues and the Robert Birley memorial lectures are a tribute to his contributions.
His biography, Red Robert: a life of Robert Birley, by Arthur Hearnden, appeared in 1984. A collection of his writings, History and Idealism: Essays, Lectures, Sermons and Letters of Robert Birley, appeared in 1990, edited by his son-in-law, Brian Rees.