The Cleveland native worked extensively on Broadway, notably in the satire on advertising, "Will Success Spoil Rock Hunter?" and continued working on stage with frequency into the 70s. But Poston's career solidified with his appearances on "The Tonight Show" (in its first incarnation with Steve Allen as host) and later as a regular on Allen's variety series. In the latter, for which he earned an Emmy, Poston played numerous characters, typically the befuddled common man on the street. At the same time, Poston was gaining name recognition through appearances on panel and quiz shows, and is particularly remembered for his long stint on "To Tell the Truth" (1985-67), as one of the two male regulars (Orson Bean was the other). In fact, for periods during the 60s, Poston was on panel shows and on stage, rarely "acting" as such on TV or in film.
Poston bounced back in the 70s. After supporting roles on two failed sitcoms, "On the Rocks" and "We've Got Each Other", he joined the hit ABC sitcom "Mork & Mindy" as Mr. Bickley, the grouchy downstairs neighbor. a role he played until the series' demise in 1982. He had made two guest appearances as Cliff 'The Peeper' Murdoch, Bob Hartley's college chum who inspired juvenile behavior in Hartley. on "The Bob Newhart Show" in the 70s. The chemistry between Poston and Newhart was palpable and when Newhart launched a new comedy series in 1982, Poston was cast as George Utley, his downeastern sidekick. He subsequently joined Howie Mandell in the short-lived "Good Grief" (Fox, 1990) a comedy set behind the scenes at a mortuary. Poston then joined Tim Conway as a foil for Conway's goofy Derk Dorf character in a CBS special and other incarnations, and made several memorable appearances as "Mr. Loon-ay," the school custodian, on ABC's "Family Matters", before joining the ABC hit series "Grace Under Fire" in 1995 as Floyd, the father of Russell (Dave Thomas).
Poston has occasionally appeared in TV longforms, such as the daffy miniseries "Fresno" (CBS, 1986). Early in his career he had played Trinculo in the 1960 "Hallmark Hall of Fame" rendition of Shakespeare's "The Tempest". Poston's feature film appearances have also been sporadic. He began on film as a detective in 1953's "The City That Never Sleeps" and perhaps is best remembered as the town cad in the 1971 Norman Lear comedy, "Cold Turkey".