Huston shot a 5-under-par 65 at the Fox Chapel Golf Club in a round highlighted by an eagle at the par-4 seventh.
The 81-man field was able to complete play despite a 5-hour rain delay.
This is the first of three consecutive majors to be played by the over-50 competitors. The U.S. Senior Open will be played in two weeks and the British Senior Open will take place two weeks after that.
Joel Edwards, Jim Gallagher, Tom Pernice, Mike Goodes and Michael Allen all had a 67 to finish the first round in a tie for sixth while the large group at 68 included Tom Lehman, Mark O'Meara, David Frost, Larry Mize and Mark Calcavecchia.
Colin Montgomerie, seven-time leading money winner on the European Tour and making his Champions Tour debut, shot a 71. Montgomerie celebrated his 50th birthday Sunday.
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