Mickelson birdied the 18th late Friday to finish a 2-over round of 72. Horschel shot 3-under 67, hitting all 18 greens in regulation at Merion Golf Club.
Their 1-under totals of 139 made them the only players under par for the tournament; nearly half the field had yet to complete the second round, which was halted because of darkness.
Luke Donald, Steve Stricker and Justin Rose were in the clubhouse at even par. Two other players were still on the course at even par.
The second round began late in the morning because storms on Thursday forced half the field to return early Friday to finish the first round.