(written from a Production point of view)
Horgan has guest-starred with fellow Star Trek performers on many other television series, including: Daniel Boone with Paul Fix; Pistols 'n' Petticoats with Stanley Adams and Robert Pine; The Girl from U.N.C.L.E. with Arnold Moss and Sabrina Scharf; Twelve O'Clock High with Frank Overton, Bert Remsen, and Paul Sorenson; Mission: Impossible with Lee Bergere and John Crawford; Cimarron Strip with George Murdock, Ron Soble, and Grace Lee Whitney; and Bewitched with Elizabeth Rogers.
He is possibly best known for his four-year stint portraying Dr. John Morrison on the soap opera The Doctors, from 1970 through 1974. He would go on to have brief stints as a regular on other soap operas, including Ryan's Hope, Guiding Light, and As the World Turns. He was also a regular on the short-lived 1983 drama Casablanca, starring David Soul, and has a role in the 1984 mini-series George Washington, along with Megan Gallagher, John Glover, Kelsey Grammer, Richard Kiley, Stephen Macht, Clive Revill, and Anthony Zerbe.
He also has a few feature film to his credit, most notably the 1968 classic heist thriller The Thomas Crown Affair and Woody Allen's 2001 comedy The Curse of the Jade Scorpion. The latter film also featured John Schuck, Wallace Shawn, and David Ogden Stiers in the cast.