Giroud, the league's leading scorer, created Geoffrey Dernis's first-half opener and made it 2-0 just after the break with his 12th goal this season before setting up John Utaka and Remy Cabella to give Montpellier 36 points from 16 games.
"We made it easy for ourselves in the second half by being very efficient," Giroud told Canal Plus TV. "I love to score but if I can create goals for my team mates, that's cool too."
Champions Lille moved up to second on 31 points after a 3-2 win at Ajaccio extended their unbeaten run to 14 games with Eden Hazard grabbing the winner with a penalty 10 minutes from time.
Stade Rennes remained in fourth on 28 points after losing 2-0 at strugglers Nice having played the second half with 10 men following defender John Boye's sending off in the 46th minute.
Montpellier again treated their home crowd to some fine, quick-passing football and there was little Lorient could do to counter it.
BEYOND DOUBT
Dernis latched on to a Giroud header to fire home in the 26th minute before the France striker was on target two minutes into the second half after Fabien Audard failed to block a Younes Belhanda free-kick.
Giroud set up Utaka in the 62nd minute as the Nigeria international put the result beyond doubt.
Second-half substitute Cabella rubbed salt in Lorient's wounds by adding a fourth from a Giroud pass four minutes from time.
Lille were made to work hard as they leapfrogged PSG into second place.
Belgian prodigy Hazard started on the bench as Rudi Garcia tried to rest him before next week's decisive Champions League game against Trabzonspor but the Lille coach was forced to send the forward on in the second half with a draw looming.
Aurelien Chedjou's own goal gave Ajaccio an early lead but Lille hit back through Ludovic Obraniak and Marko Basa in the 12th and 16th minutes respectively.
Ilan equalised for the hosts eight minutes before the break but Hazard popped up to score a late winner from the spot after midfielder Florent Balmont was fouled.
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