Legg was last seen on July 10, 1971 in Oswego County, New York. He was visiting his uncle at the family's 12,500-acre estate in the Santonini Preserve in Newcomb at the time. On the day of his disappearance, Legg went on a family hike in the Adirondack wilderness. His uncle sent him back home to change into long pants at 3:30 p.m. Legg had gone hiking many times in the local area and was very familiar with it, and it was only a short distance back to camp. He never arrived there, however, and has never been heard from again.

An extensive search, lasting six weeks and largely financed by Legg's wealthy family, turned up no indication of his whereabouts. He was in the third grade at the time of his disappearance, and resided in Baldwinsville, New York. His loved ones describe him as a happy, healthy, normal child. Although there were rumors that foul play was involved in his case, most investigators believe Legg became lost or died in an accident in the wilderness. His case remains unsolved.