After Amherst, he spent time at Yale Divinity before earning an M.A. from Columbia University in American History in 1958. Allan then taught American history for 33 years at Horace Greeley High School in Chappaqua, N.Y., retiring in June 1991. Upon retirement, he completed his Ph.D. in American History from Columbia in 1994 and spent many years as a trustee and treasurer of the National Council for History Education. In addition, he was a researcher, writer and contributing editor for American Heritage and a contributing writer to The Dictionary of American Biography and to Scribner's Encyclopedia of American Lives.