McMahon will take over for Steve Hoffman, who wasn't retained after Romeo Crennel was elevated from interim head coach to the permanent job. Also let go were wide receivers coach Richie Anderson and assistant offensive line coach Pat Perles.
"I'm very blessed to have the opportunity to come to Kansas City and be a part of Romeo's staff," said McMahon, who spent the past three seasons coaching special teams for St. Louis.
"Romeo and I have a shared vision of what it takes to win, and I'm excited to get started."
Crennel has already announced that he will serve as his own defensive coordinator, though he hasn't announced who will be the offensive coordinator.
Longtime assistant coach Bill Muir, who served as offensive coordinator last season, has talked about retirement after struggling in the job.
Crennel has said he likely won't announce his choice until after the Super Bowl.
Copyright 2011 by The Associated Press