British writer of detective stories, mostly featuring the medico-legal forensic investigator Dr. Thorndyke.

His first stories were written in collaboration with Dr. John James Pitcairn [1860–1936], medical officer at Holloway Prison and published under the nom de plume "Clifford Ashdown". His first Thorndyke story, The Red Thumb Mark, was published in 1907 and shortly afterwards he pioneered the inverted detective story, in which the identity of the criminal is shown from the beginning: some short stories with this feature were collected in The Singing Bone in 1912. During the First World War he served as a Captain in the Royal Army Medical Corps and afterwards produced a Thorndyke novel almost every year until his death in 1943.