In 1935, at the suggestion of Panofsky, who had emigrated to the United States, Alfred Barr sponsored Janson as an immigrant, and he completed a PhD at Harvard University in 1942 (his dissertation was on Michelozzo).

In 1941 he married Dora Jane Heineberg (1916–2002), an art history student at Radcliffe College who later collaborated with him as co-author, and he became a citizen in 1943.

Janson's plan to sell popular canvases such as Frederic Remington's A Dash for the Timber at the New York galleries of the Kende family drew comment from the local paper, wondering why St. Louisans had not been given preference.

[4] Janson sold 120 artworks, retained 80, and acquired 40 works by European modernists through the Kende Galleries:[5] Paul Klee, Juan Gris, Theo van Doesburg.

"[8] The updated editions of his History of Art, made by his son, Anthony F. Janson, have included several women artists from different eras.