After graduating from Harvard Law School in 1865, Fiske briefly practiced law in Boston before turning to writing. In 1860 he had encountered Herbert Spencer's adaptation of the evolutionary theory of Charles Darwin to aspects of philosophy. Deeply impressed by their ideas, he attempted to incorporate them into his own writings. A visit to Europe (1873–74) provided him the opportunity to meet and talk at length with Darwin, Spencer, and T.H. Huxley. The result was the publication, in 1874, of Fiske's Outlines of Cosmic Philosophy, an exposition of evolutionary doctrine that was well received both at home and abroad. About 1880 his interests turned to American history as interpreted in the light of evolutionary theory, and from 1885 to 1900 he lectured and published voluminous works on the American colonial and revolutionary periods.

The same belief in inevitable progress through evolutionary change prevailed in Fiske's interpretation of American history in such works as The Critical Period of American History, 1783–1789 (1888). His primary contribution to American thought was popularizing the evolutionary thesis against the adamant opposition of the churches, however.