The two actors succeed Willemijn Verkiak and Kyle Dean Massey, who played their final performances May 26.
They join a cast that includes Katie Rose Clarke as Glinda, Randy Danson as Madame Morrible, Adam Grupper as The Wizard, Catherine Charlebois as Nessarose, F. Michael Haynie as Boq and John Schiappa as Dr. Dillamond.
Mendez appeared on Broadway as the "Bless the Lord" soloist in the 2011 revival of Godspell. Other credits include Everyday Rapture (both on Broadway and in its Off-Broadway world premiere), Grease (2007 Broadway revival) and The Marvelous Wonderettes (Off-Broadway).
Klena, who is making his Broadway debut, was also recently seen in Carrie at MCC.
Adapted from the novel by Gregory Maguire, Wicked delves into the back story of the iconic characters from Oz. The hit musical has a score by Stephen Schwartz, a book by Winnie Holzman, direction by Mantello and musical staging by Wayne Cilento.
The production also features scenic design by Eugene Lee, costume design by Susan Hilferty, lighting design by Kenneth Posner, sound design by Tony Meola, projections by Elaine J. McCarthy, wig and hair design by Tom Watson, music supervision and arrangements by Stephen Oremus, orchestrations by William David Brohn and dance arrangements by James Lynn Abbott.
For tickets visit WickedtheMusical.