Kates had previously made guest appearances on a number of classic television programs, including two episodes of The Untouchables with Nehemiah Persoff and Jason Wingreen. Kates also appeared with Persoff in a 1961 episode of Bus Stop. The same year, Kates guest-starred on Alfred Hitchcock Presents in the episode "Outlaw in Town". Guest-starring in this episode were TOS guest actors Clegg Hoyt and Ricardo Montalban.
Other television series on which Kates co-starred with other Star Trek performers include:
- Johnny Staccato episode "Double Feature" with Garry Walberg (1960)
- Playhouse 90 episode "In the Presence of Mine Enemy" with Celia Lovsky (1960)
- Michael Shayne episode "It Takes a Heap o' Dyin'" with Yvonne Craig (1961)
- Sam Benedict episode "Maddon's Folly" with Robert Lansing (1962)
- Dr. Kildare episode "The Eleventh Commandment" with Michael Forest and Susan Oliver
- The Outer Limits episode "Nightmare" with David Frankham, John Anderson, Whit Bissell, Willard Sage and Vic Perrin, make-ups by Fred Phillips, written and produced by Joseph Stefano and directed by John Erman with Robert H. Justman as assistant director
Kates had a three-year stint playing Ben Scott on the soap opera Guiding Light, from 1965 through 1968. He was part of the original cast of the TV drama Where the Heart Is, as was Laurence Luckinbill. More recent television works include guest spots on Gabriel's Fire (starring Madge Sinclair), NYPD Blue (starring Sharon Lawrence and Gordon Clapp), and 3rd Rock from the Sun (in an episode with David Youse). He also had a role in the 1990 made-for-TV movie Angel of Death, along with fellow Next Generation guest stars Brian Bonsall, John de Lancie, and Ray Walston.
Kates appeared as a reporter in the classic film Judgment at Nuremberg in 1961. TOS star William Shatner had a role in this film. Kates' more recent film credits include The Babe (1992, with James Cromwell, Danny Goldring, and Larry Cedar) and The Phantom (1996, with Cary-Hiroyuki Tagawa, Bill Smitrovich, Samantha Eggar, Leon Russom).