William Cahill was born in Syracuse, New York. He moved to Wilmington to receive criticism from Howard Pyle in the fall of 1907; he was not a full-time student. In a letter to his mother, dated November 1907, N.C. Wyeth described Cahill, as a new member of the colony who had ideas dramatically opposed to illustration. According to Wyeth, Cahill was from, “clique” of New York artists including John Alexander, Birge Harrison, Chiide Hassam and H.D. Murphy.

After spending the summer of 1908 at Chadds Ford painting with Howard E. Smith and N.C. Wyeth, Cahill returned to New York in the fall. In 1918 he became a professor of drawing, and painting, at the University of Kansas. Cahill was a member of the Salmagundi Club in New York.

Examples of his work are in the collections of the Museum of History, Science and Art in Los Angeles, California and the Municipal Collection, Pheonix, Arizona.