Trevor Williams realistically only had to give the Mets a few innings Tuesday and depart with the team still in the game to consider it mission accomplished.

The right-hander, in a start to replace injured Tylor Megill, instead gave his team the bonus package: four shutout innings against the Cardinals that allowed the Mets to build a lead they held throughout in their 3-1 victory in Game 1 of a doubleheader at Citi Field.

The Mets cleansed their palates from the distaste of losing their first series of the season over the weekend against the Mariners.

Williams, in his second start of the season, allowed four hits with six strikeouts over 65 pitches. In his previous start of the season he got jumped by the Diamondbacks over two innings, allowing four runs.

“I’m happy to contribute, I know this is what my job is, to save the bullpen,” Williams said.

Last week, Williams pitched 3 ²/₃ innings of scoreless relief after Megill was knocked out early against the Nationals.

“Learning this new role has been a fun challenge for me,” Williams said. “I’ve had a lot of guys in my corner and I have reached out to guys around the league who have been in this role before, knowing what to do: not starting anymore and kind of like preparing and staying ready to pitch every day.”

Williams credited backup catcher Patrick Mazeika, who started his second game since James McCann was placed on the injured list with a fractured hamate bone in his wrist, for keeping him steady.

“The pitchers are familiar with [Mazeika] and the guys that aren’t are feeding off what guys are telling them too,” manager Buck Showalter said.

Jeff McNeil and Dominic Smith each delivered an RBI double in the third inning that extended the Mets’ lead to 3-0. Francisco Lindor reached on Brendan Donovan’s error to begin the inning, leaving one run unearned for Miles Mikolas.

The Mets loaded the bases to begin the second against Mikolas and scored a run on Travis Jankowski’s RBI fielder’s choice.

Jake Reed replaced Williams for the fifth and walked two batters, but escaped without allowing a run by striking out Paul Goldschmidt and retiring Nolan Arenado. Reed returned to work a perfect sixth inning. Seth Lugo worked a perfect seventh before Drew Smith allowed a solo homer to Goldschmidt in the eighth and Edwin Diaz pitched the ninth.

For Smith it was a third straight appearance allowing a run after starting the season with 13 ¹/₃ scoreless innings over 12 outings. Diaz allowed a single and walk in the ninth before striking out Harrison Bader to end it. The strikeout was Diaz’s third of the inning.

“Later in the year I probably would have pitched [Diaz] in the eighth inning instead of the ninth,” Showalter said after the Cardinals sent the top of the order up in the eighth. “There were a couple of things there [in the ninth], but he keeps grinding and keeps working … but it doesn’t go unnoticed, Edwin has been instrumental in the job we have done early on.”

Before each at-bat Arenado was booed for his role in an April 27 benches clearing incident in St. Louis that involved the Mets. In that game, Yoan Lopez threw high-and-inside to Arenado, who started toward the mound. After the benches emptied and shoves were exchanged by the teams, Pete Alonso was tackled from behind by Cardinals first-base coach Stubby Clapp.