McMahon is rated the No. 7 third-base prospect in the Minors by MLBPipeline.com. But with the Rockies set at third base with two-time All-Star Nolan Arenado, McMahon split his time between first and third at Double-A Hartford (67 appearances at each position) and has done the same in the AFL.
McMahon, 21, is in his first AFL action. He hit .242 with 12 home runs and 75 RBIs at Hartford, and is batting .214 with a triple and four RBIs for the Salt River Rafters in the AFL.
Fans will add two more players via the Final Two Fan Vote. The Cubs' Ian Happ (No. 21 overall), the Mets' Gavin Cecchini (No. 3 in the organization) and the Yankees' Miguel Andujar (No. 7 in the organization) are the candidates for the East roster, while the West has Red Sox's No. 12 prospect Mauricio Dubon, White Sox No. 14 Courtney Hawkins and the Reds' Brandon Dixon on the ballot.
Last year's contest displayed several players who made an impact in the Majors this year, including Yankees catcher Gary Sanchez, Cardinals right-hander Alex Reyes and Cubs backstop Willson Contreras.
Thomas Harding has covered the Rockies since 2000, and for MLB.com since 2002. Follow him on Twitter @harding_at_mlb, listen to podcasts and like his Facebook page. This story was not subject to the approval of Major League Baseball or its clubs.