Life with Unix says: Wrote sed.

A Research Unix Reader says: Lee E. McMahon's linguistic insight fostered the characteristic text-processing — as distinct from text-formatting — capabilities of the system. He wrote comm. qsort, sed, the current grep. and the concordance-builders index for English and cref for C. After an early fling with cu and an influential laboratory switching system done with Condon, Morris, Thompson, Chuck (Charles B.) Haley and Cherry, Lee became the prime software architect for Sandy Fraser's Datakit* switch.