This rhyming phrase has long been associated with Errol Flynn, an Australian actor who had a reputation for womanizing, drinking, and brawling.
In November 1942, two under-age girls accused him of statutory rape. A group was organized to support Flynn, named the American Boys’ Club for the Defense of Errol Flynn (ABCDEF); its members included William F. Buckley, Jr. The trial took place in January and February 1943, and Flynn was cleared of the charges.
According to etymologist Michael Quinion, the incident served to increase Flynn’s reputation as a ladies’ man, which influenced the connotations of the phrase “in like Flynn.” Columnist Cecil Adams also examined the term’s origins and its relationship to Flynn. Many early sources say it emerged as war slang during World War II.
Barry Popik of the American Dialect Society found an example from 1940, as well as this from the sports section of the San Francisco Examiner of 8 February 1942: “Answer these questions correctly and your name is Flynn, meaning you’re in, provided you have two left feet and the written consent of your parents.”
In addition to the Errol Flynn association, etymologist Eric Partridge presents evidence that it refers to Edward J. Flynn, a New York City political boss who became a campaign manager for the Democratic party during Franklin Delano Roosevelt’s presidency. Boss Flynn’s “Democratic Party machine exercised absolute political control over the Bronx…. The candidates he backed were almost automatically ‘in’.”
The word “in” had been used with regard to success, good fortune or sexual conquest for some years prior to the 1940s; for example: Alfred Mason’s Clementina, 1901: “His luck for the moment was altogether in.”
Considering the prior use of “in” and human delight in rhyming phrases, I can imagine that “in like Flynn” arrived in the language before Errol Flynn arrived in America.
But he certainly gave it some pizzazz!