Kilroy was here.

Wiki:

"James J. Kilroy, an American shipyard inspector, was the man behind the signature. During World War II he worked at the Bethlehem Steel 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 they 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 omnipresence and inscrutability sparked the legend. Afterwards, servicemen could have begun placing the slogan on different places and especially in new captured areas or landings. At some later point, the graffiti (Chad) and slogan (Kilroy was here) must have merged. (Michael Quinion. 3 April 1999.[1])
The New York Times reported this as the origin in 1946, with the addition 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 what else he could do?

However, The Times also notes that James J. Kilroy's story only came to light as a result of a contest to find the originator of the phrase; the contest was sponsored by The American Transit Association. The article makes no mention of how the contest was decided, or how credible Mr. Kilroy's story was deemed to be.

Another contender stepped forward a year before James J. Kilroy, when Sgt Francis J Kilroy claimed that he was the originator of the phrase, having scribbled something similar on a bulletin board that was then shipped overseas.[2]
Author Charles Panati says, “The mischievous face and the phrase became a national joke.” He continued to say, "The outrageousness of the graffiti was not so much what it said, but where it turned up."
While the origins of the slogan are obscure, those of the cartoon are less so. It almost certainly originated as "Chad", in the UK before the war; a creation of the cartoonist George Edward Chatterton. Presumably, the two merged together during the 1940s, with the vast influx of Americans into Britain. The "Chad" cartoon was very popular, being found across the UK with the slogan "What, no …?" or "Wot, no …?" underneath, as a satirical comment on shortages and rationing. (One sighting, on the side of a British 1st Airborne Division glider in Operation Market Garden, had the plaintive complaint "Wot, no engines?"). Later, as the country began to prosper in the 1950s and 1960s, it became a feature of some forms of advertising, especially on posters touting home improvements etc. For instance in many areas of the country outdoor toilets were the norm, so a poster might say "Wot, no inside lav?" advertising indoor plumbing.
Kilroy was the most popular of his type in World War II, as well as today. Clem (Canadian), Overby (Los Angeles- late 1960s), Chad (British- WW II), and Mr. Foo (Australian- WW I & II) never reached the popularity Kilroy did. The ‘major’ Kilroy graffito fad ended in the 1950s, but today people all over the world scribble ‘Kilroy was here’ in schools, trains, and other similar public areas.
Kilroy is still known and used today by US Servicemen. He has been seen scribbled on barriers on Main Supply Routes (MSRs) in Iraq and on warehouses in Taji, Iraq.

Hello Kilroy from experimentaljetset.nl

There are many legends attached to the Kilroy graffiti. One states that Adolf Hitler believed that Kilroy was some kind of American super spy because the graffiti kept turning up in secure Nazi installations, presumably having been actually brought on captured Allied military equipment. Another states that Stalin was the first to enter an outhouse especially built for the leaders at the Potsdam conference. Upon exiting, Stalin asked an aide, "Who is this Kilroy?" Another legend states that a German officer, having seen frequent "Kilroys" posted in different cities, told all of his men that if they happened to come across a "Kilroy" he wanted to question him personally.

The graffiti is supposedly located on various significant and/or difficult-to-reach places such as on the torch of the Statue of Liberty, on the Marco Polo Bridge in China, in huts in Polynesia, on a high girder on the George Washington Bridge in New York, at the peak of Mt. Everest, on the underside of the Arc de Triomphe, scribbled in the dust on the moon, in WWII pillboxes scattered around Germany, around the sewers of Paris, and, in tribute to its origin, engraved in the WWII Memorial in Washington D.C.
The Transit Company of America held a competition in 1946 offering a real trolley car to the man who could verify he was the "real Kilroy". J. J. Kilroy brought his co-workers with him to prove that he was undeniably the true Kilroy. The other forty or so men who showed up were not able to establish they were the "real" Kilroy. Kilroy gave his prize to his nine children to play with in their front yard