Borle will recur in the guest starring role of Frank Masters, William Masters' brother, portrayed on the series by Michael Sheen.
The series also stars Lizzy Caplan, Caitlin Fitzgerald, Teddy Sears and Annaleigh Ashford.
"Masters of Sex," is based on "real-life pioneers of human sexuality, William Masters and Virginia Johnson, the series chronicles their unusual lives, romance and pop culture trajectory. Their research touched off the sexual revolution and took them from a mid-western teaching hospital in St. Louis to the cover of Time magazine. The series is an adaptation of Thomas Maier's book 'Masters of Sex: The Life and Times of William Masters and Virginia Johnson, The Couple Who Taught America How To Love.'"