factor first appeared on 5th edition Research Unix in 1974, as a "user maintained" utility (section 6 of the manual). In the 7th edition in 1979, it was moved into the main "commands" section of the manual (section 1). From there, the factor utility was copied to all other variants of Unix, including commercial Unixes and BSD. In some variants of Unix, it is classified as a "game" more than a serious utility, and therefore documented in section 6.
A free software version of the factor utility was written for the GNU project by Paul Rubin, in 1986. It is now available on all Linux distributions as part of the GNU Core Utilities. In 2008, GNU factor started to use GNU MP library for arbitrary-precision arithmetic, allowing it to factor integers of any size, not limited by the machine's native datatypes.