Lacazette, who was booed by the crowd before kickoff because he plays for St Etienne's bitter rivals Olympique Lyonnais, was then loudly cheered when he scored France's first in the 14th minute.
Giroud doubled their tally before the interval as Les Bleus, the Euro 2016 hosts, cantered to a win that showed their strength in depth.
Denmark, who beat the United States 3-2 in a friendly on Wednesday in Aarhus, were much better in the second half with Nicklas Bendtner being denied by Stephane Ruffier's brilliant save in the 65th minute.
They lie second in their Euro qualifying Group I with seven points from four games, two adrift of Portugal.
France coach Didier Deschamps made eight changes to the team who lost 3-1 against Brazil, with only centre back Raphael Varane, midfielder Morgan Schneiderlin and forward Antoine Griezmann left from Thursday's friendly at the Stade de France.
"I saw good things in the first half but after the break we struggled. Maybe it's normal after we made some more changes. Anyway, we stayed focused and that's good," Deschamps told French TV channel TF1.
Lacazette seized the opportunity and scored his first goal in six caps with a half volley from inside the box after Kasper Schmeichel had deflected Griezmann's shot into the path of the Ligue 1 top scorer.
Lacazette came close again in the 24th minute but Schmeichel superbly parried away his 20-metre thunderbolt.
Seven minutes before the break, Giroud was nicely played in by Geoffrey Kondogbia before beating Schmeichel with a low shot.
Denmark stepped up a gear after the break and Ruffier made a superb save to deny Bendtner and preserve France's lead as the visitors dominated possession.
Morten Olsen's team increased the pressure but the French defence, marshalled by Varane who was captain on the day in the absence of the injured Hugo Lloris, held on.
Ruffier then dived at the feet of Bendtner in the 88th minute after the former Arsenal striker had slalomed his way into the box.
France next face Belgium at home and Albania away in friendlies in June, when Denmark will take on Serbia in a Euro qualifier.
(Reporting by Julien Pretot; Editing by Mark Meadows and Toby Davis)
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