Grosenick made 33 saves in his first NHL game since Nov. 18, 2014, the sixth-longest gap between NHL starts by a goalie in NHL history (6 years, 102 days).
"He just wanted an opportunity, maybe get a game in here or there," Kings coach Todd McLellan said of Grosenick. "Tonight it presented itself and he took advantage of it, so really good for him."
Adrian Kempe scored two goals to give him five in the past two games for the Kings (11-8-6), who split the two-game set after losing 6-5 in overtime to the Ducks on Monday.
Sam Steel scored, and John Gibson made 32 saves for the Ducks (8-13-6), who were trying to win three straight for the first time this season.
"I don't think we made it nearly as difficult on him as we could have," Ducks defenseman Cam Fowler said of Grosenick.
Grosenick learned Wednesday afternoon that he would likely start after Calvin Petersen was placed in NHL COVID-19 protocol prior to the game.
The 31-year-old said he wasn't affected by the short notice.
"Just same old thing, you know. Just take the next shot as it comes, just take the next day as it comes," he said. "If you're prepared for anything, things don't take you by surprise as much."
Kempe, who had his second NHL hat trick on Monday, gave the Kings a 3-1 lead when he scored off a feed from Trevor Moore at 17:55 of the second period for his 11th goal of the season.
Andreas Athanasiou made it 4-1 when he pushed in a rebound at 1:27 of the third period.