The drama, which is set in the fashion world, is being executive produced by Grey's Anatomy's Mark Gordon.
Anthony LaPaglia has already signed up to play fashion designer Robert Soulter, the patriarch of the family featured in the show.
Olin has now joined the cast as Martin, the owner of a company which makes clothes, The Hollywood Reporter says. Martin's daughter works for the company.
Meanwhile, Mendoza will play Wendy, who is described as pretty but mean.
Olin has previously starred in shows such as Hill Street Blues, thirtysomething and Brothers & Sisters, while Mendoza appeared in the British drama Hotel Babylon and was due to appear in the Broadway musical Spider-Man: Turn Off the Dark but left after an injury.
Americana is being penned by North Country scribe Michael Seitzman.