Fritsch, the son of a London headmaster, was educated at the University of London and at Munich, where he obtained his DPhil in 1899. He taught at University College, London, from 1902 until 1911, when he moved to Queen Mary College, London, where he served as professor of botany from 1924 until his retirement in 1948.

Before Fritsch there was no comprehensive work on algae. He remedied this with his classic work The Structure and Reproduction of the Algae (2 vols. 1935–45). He was also instrumental in the foundation of the Freshwater Biological Association in 1929.