Freeman made his lasting mark with The Red Thumb Mark (1907), which introduced Dr. Thorndyke, a medical jurist whose investigations emphasized scientific method and forensic evidence—fingerprints, blood analysis, and document examination—setting a new standard for the genre. He also pioneered the "inverted detective story," in which the crime and criminal are revealed at the outset and the interest lies in how the detective solves the case; The Singing Bone (1912) collects these tales.
Among his most notable works are The Eye of Osiris (1911), The Mystery of 31 New Inn (1912), A Silent Witness (1914), Helen Vardon's Confession (1922), Mr. Pottermack's Oversight (1930), and The Jacob Street Mystery (1942). Freeman died in 1943, leaving behind a legacy as a founding father of the forensic mystery.