With `Sammy' Eilenberg, Saunders Mac Lane was one of the original pioneers of category theory. He initially worked on it as a language to enable ‘natural transformations’ to be described in a ‘natural’ way, and also developed, again with Eilenberg many of the strong links with group theory and the cohomology of groups. He was the author of one of the key books on homological algebra, see below.
With Henry Whitehead he gave the first algebraic description of the homotopy 2-type of a space.
Homology, number 114 in Grundlehren, Springer 1967 (pdf)
Mathematics form and function, Springer 1986
Selected Papers. Edited by I. Kaplansky. Springer 1979
(with J. H. C. Whitehead) On the 3-type of a complex, Proc. Nat. Acad. Sci. U.S.A., 36, (1950), 41 – 48 (N.B. Their 3-type is the modern 2-type.)
Geometrical Mechanics, Lectures 1968 (web)