Harman, 17, of Savannah, Georgia, leads Matt Savage by one stroke, as first- round play at the Lake Course has been suspended until Tuesday.
Play was delayed for three hours due to heavy morning fog, leaving a large portion of the 156-player field unable to complete their first rounds.
'It was tough to give that trophy back,' said Harman, who became only the third lefty to win a USGA title. 'That's mine. I want it back.'
Savage turned out a 15-stroke improvement from last year's opening round.
'I've made things a lot easier for tomorrow,' said Savage. 'I just hope I don't go the other way. I feel like I belong here.'
Daniel Lim sits in third place after shooting a 1-over 71. David Chung, Marshall Pickett and Matthew Swan are tied for fourth at 2-over 72, while Bradley Johnson, Daniel Woltman and David May sit six strokes off the lead at 3-over 73.
Following 36 holes of stroke play over the 6,790-yard, par 70 layout that has been host to four U.S. Opens, the top 64 scorers in this age 17-and-under Championship will advance to match play beginning on Wednesday. Play runs through Saturday's 18-hole final match.