Toffoli's second goal late in the middle period for a 3-0 lead gave Los Angeles what turned out to be the decisive score.
Jarret Stoll also scored as the Kings bested the Blues in their first meeting since last year's Western Conference quarterfinals, which Los Angeles won 4-games-to-2.
Jaroslav Halak took a shutout late into the game before yielding goals to Kevin Shattenkirk and Vladimir Tarasenko in the final minutes, but held on for the victory with 20 saves.
Ben Scrivens turned aside 25 shots for St. Louis, which dropped a second straight following a five-game winning streak.
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