His childhood was spent in Cambridge, Massachusetts, where he attended Shady Hill School.
His father was a historian of Asian decorative arts, founder of the Captain Robert Bennet Forbes House and curator of Asian export art at the Peabody Essex Museum in Salem, Massachusetts.
From 1998 to 2006, Forbes served as the founding associate director of Yale's Gilder Lehrman Center for the Study of Slavery, Resistance, and Abolition, under the directorship of David Brion Davis.
[citation needed] Between 2006 and 2014, Forbes taught U.S. history and American Studies at the University of Connecticut, Torrington.
His first book, The Missouri Compromise and its Aftermath: Slavery and the Meaning of America, the first major work on the subject in over fifty years, was described as "a profound study" by Oxford historian Daniel Walker Howe.