McMahon was optioned to Triple-A Albuquerque on Tuesday, Thomas Harding of MLB.com reports.
McMahon hit just .180/.317/.200 in 60 plate appearances for the Rockies, striking out 36.7 percent of the time. Better things were expected from one of the Rockies' top prospects, but he never got the consistent playing time he needed to get his bat going. Daniel Castro was called up to take his place on the roster.