Mr. Keen, Tracer of Lost Persons, was a popular radio detective show that aired from October 12, 1937, to April 19, 1955. Originating from the creative minds of Frank and Anne Hummert, the series follows the adventures of the "kindly old investigator" Mr. Keen and his assistant, Mike Clancy, as they track down missing persons and solve murders. The show's title character was played by Bennett Kilpack, Arthur Hughes, and Philip Clarke at various times, while Jim Kelly portrayed Clancy.

Initially a thrice-weekly fifteen-minute serial, the show transitioned to a half-hour weekly format in 1943. Despite its melodramatic and heavy-handed approach, Mr. Keen, Tracer of Lost Persons enjoyed a dedicated following and even inspired parodies by comedy duo Bob and Ray, as well as being referenced in pop culture.

Having aired on both the NBC Blue network and later CBS, the series had numerous sponsors, including Whitehall Pharmacal, Kolynos Toothpaste, BiSoDol antacid mints, and Chesterfield cigarettes. Today, around 60 of its 1690 episodes survive for modern audiences to enjoy.