The Oxford English Dictionary says simply that Kilroy was “The name of a mythical person”. One theory identifies James J. Kilroy (1902–1962), an American shipyard inspector, as the man behind the signature. The New York Times indicated J.J. Kilroy as the origin in 1946, based on the results of a contest conducted by the Amalgamated Transit Union to establish the origin of the phenomenon. The article noted that Kilroy had marked the ships themselves as they were being built—so, at a later date, the phrase would be found chalked in places that no graffiti-artist could have reached (inside sealed hull spaces, for example), which then fed the mythical significance of the phrase—after all, if Kilroy could leave his mark there, who knew where else he could go? Brewer’s Dictionary of Phrase and Fable notes this as a possible origin, but suggests that “the phrase grew by accident.”
During World War II he worked at the Fore River Shipyard in Quincy, Massachusetts, where he claimed to have used the phrase to mark rivets he had checked. The builders, whose rivets J.J. Kilroy was counting, were paid depending on the number of rivets they put in. A riveter would make a chalk mark at the end of his or her shift to show where he had left off and the next riveter had started. Unscrupulous riveters discovered that, if they started work before the inspector arrived, they could receive extra pay by erasing the previous worker’s chalk mark and chalking a mark farther back on the same seam, giving themselves credit for some of the previous riveter’s work. J.J. Kilroy stopped this practice by writing “Kilroy was here” at the site of each chalk mark. At the time, ships were being sent out before they had been painted, so when sealed areas were opened for maintenance, soldiers found an unexplained name scrawled. Thousands of servicemen may have potentially seen his slogan on the outgoing ships and Kilroy’s apparent omnipresence and inscrutability sparked a legend. The slogan began to be regarded as proof that a ship had been checked well, and as a kind of protective talisman. Afterwards, servicemen began placing the slogan on different places and especially in newly captured areas or landings, and the phrase took on connotations of the presence or protection of the US armed forces.
The Lowell Sun reported in November 1945, with the headline “How Kilroy Got There”, that a 21-year old soldier from Everett, Sgt. Francis J. Kilroy, Jr., wrote “Kilroy will be here next week” on a barracks bulletin board at a Boca Raton airbase while ill with flu, and the phrase was picked up by other airmen and quickly spread abroad. The Associated Press similarly reported at the same time that according to Sgt. Kilroy, when he was hospitalized early in World War II a friend of his, Sgt. James Maloney, wrote the phrase on a bulletin board. Maloney continued to write the shortened phrase when he was shipped out a month later, and other airmen soon picked up the phrase. Francis Kilroy himself only wrote the phrase a couple of times.