Guyer is a busy television actor, with appearances on Law & Order, The Sopranos, Oz (four episodes as Officer Hunt in the first season), 24, Homicide: Life on the Street, Ed, Blue Boods, House of Cards, Person of Interest, Elementary, White Collar, and Billions. He played Kate Winslet's father-in-law in the HBO miniseries Mildred Pierce. Guyer has been a frequent voice-over actor in Ken Burns documentaries, as William Clark in Lewis & Clark: The Journey of the Corps of Discovery and other historical voices in Horatio's Drive, Unforgivable Blackness, and The National Parks: America's Best Idea. In video-games, he voiced characters in Neverwinter NIghts 2 and Red Dead Redemption.
Guyer has performed on Broadway in both musicals (as Captain Orton in the 2015 revival of The King and I, a replacement as Captain Brackett in South Pacific) and dramatic plays (as Doc Golightly in the stage version of Breakfast at Tiffany's), as well as off-Broadway (The Misanthrope in 1995). Guyer is also a playwright, author of Eden Court and World of Mirth (both staged off-Broadway). Film credits include City Hall (with Martin Landau and Fran Brill), The Peacemaker (with Nicole Kidman), Rounders, The Devil's Advocate, and the 2011 remake of Arthur.