In the Dragnet episode "The Big Prophet" (1968), Sullivan as his character Brother William (a thinly disguised portrayal of Timothy Leary) held forth for the entire half-hour on the benefits of LSD and marijuana while Joe Friday argued the opposing view.

In 1966–67 season, he appeared as Major Mapoy in all twenty-six episodes of the ABC Western series The Monroes with Michael Anderson, Jr., and Barbara Hershey.

Two years later he was back with Shatner, this time performing the role of Parmen, a supremely arrogant and cruel telekinetic alien, in the Star Trek episode "Plato's Stepchildren" (1968).

[6] In the latter episode S2 E25, which aired in 1961, he played a youngish and overly loquacious member of a gentleman's club who accepts a challenging and rather bizarre bet offered by an older member of the club (played by actor Franchot Tone) that ends in unintended consequences.

Telepathy was the subject of experiments Sullivan conducted in the documentary film from 1977 called The Amazing World of Psychic Phenomena hosted by Raymond Burr.

When he died, Sullivan was working on two books: a novel and the history of his family's Eli Bridge Co., which was credited with constructing one of the first Ferris wheels.