The new Broadway musical also stars Lisa O'Hare and Lauren Worsham.
Sam Harris and Brenda Braxton were also on hand to perform from their future shows.
Original Jean Valjean Colm Wilkinson shared the stage with Karimloo in a Toronto performance of Les Misérables.
Jerry Seinfeld, Bernadette Peters, and Clive Davis were also on hand to cheer for Jessie Mueller and the cast of the new musical.
The Tony Award winner met with the cast following the Sunday, January 12 matinee.
The new romantic comedy beings previews at Playwrights Horizons on February 7.
Photos from the festival's opening-night celebration.
Two-time Tony Award winner Stephen Spinella costars in the upcoming Broadway production.
Catch a glimpse of the pair as their Steinbeck characters in the image soon to hit the marquee at the Longacre Theatre.
The singer took in a concert on Monday night.