Giroud headed home with seven minutes left as Montpellier, who have 24 points from 14 games, leapfrogged Paris Saint-Germain, 2-1 winners against Caen.
Previous leaders Brest dropped to fifth only two points off the pace after losing 2-1 at third-placed Stade Rennes.
"I just benefited from the work of my team mates," Giroud told French TV channel Canal Plus.
"We played a solid game and we had more chances to score so we deserve our victory."
Montpellier's win ended Paris Saint-Germain's brief stay at the top of the standings following their home win against Caen earlier in the day.
France forward Hoarau headed home Siaka Tiene's cross from the left to put the hosts ahead five minutes before the break.
Didier Deschamps's side are just one point behind PSG and Rennes, who handed promoted visitors Brest their second defeat in three league games.
Brest midfielder Bruno Grougi reduced the arrears from the spot in added time but it was too late for a comeback after Kembo-Ekoko's curled shot and Leroy's powerful strike from inside the box had put Rennes firmly in charge.
