Guy L. Steele Jr. and Richard P. Gabriel (1993) name him as a leader of the Common Lisp movement and describe him as "a seductively powerful thinker, quiet and often insulting, whose arguments are almost impossible to refute".

[1] Maclisp, a variant of Lisp developed at the Massachusetts Institute of Technology (MIT) by Richard Greenblatt in the late 1960s, originally ran on the PDP-6 and PDP-10 computers made by Digital Equipment Corporation.

In the early 1970s, Moon headed a project at MIT that reimplemented Maclisp on a different kind of computer, the Honeywell 6180 running the Multics operating system.

[3] In 1976, with Steele, he wrote the first (TECO-based) version of the Emacs text editor,[4][5] and in 1978 with Daniel Weinreb he coauthored the manual for the Lisp Machine, known as the chine nual.

With Howard Cannon, he developed Flavors, a system for doing object-oriented programming with multiple inheritance on the Lisp Machine.