He was educated at Abingdon School in Oxfordshire and served in the British Army from 1940–1946, achieving the rank of major. After the war he obtained a BSc in Astronomy, and then took an administrative post at the secretariat of The Royal Society. While there he did an MSc in the History and Philosophy of Science under Herbert Dingle at University College London. After leaving the Royal Society he took up writing, and during a long career as an author produced over forty books, mainly on astronomy, and the history and philosophy of science. Later in life he collaborated with Joseph Needham on an abridgement of Needham's great work on China, producing The Shorter Science and Civilization in China in several volumes. He played key roles in the administration of the British Astronomical Association, where he was president from 1989 to 1991, and for many years he was the editor of its journal, and director of the historical section.
With his second wife Ann, he founded the Ronan Picture Library, which specialises in scientific and historical pictures. Among his many books on the history of science were studies of scientists such as Galileo, William Herschel and Edmond Halley. He also wrote scientific books for children, along with books such as The Practical Astronomer (1981) written for beginner amateur astronomers.
Ronan had an asteroid named in honour of his achievements: 4024 Ronan belongs to the Floras family, discovered by E. Bowell on November 24, 1981, at Anderson Mesa.