Variety reported the casting, but said the roles Cain and Slater will play haven't been revealed yet.
The actor and actress also previously guest-starred on various episodes of the Superman origins TV drama Smallville.
The Supergirl series will star Melissa Benoist as the titular heroine, also known as Kara Zor-El. Mehcad Brooks will play James "Jimmy" Olsen and Calista Flockhart will play Cat Grant. Laura Benanti, Chyler Leigh and David Harewood also recently joined the cast.
Greg Berlanti and Allison Adler are to serve as co-writers and executive producers on the show.