Poston was born in Columbus, Ohio, the son of George and Margaret Poston; his father was a liquor salesman and dairy chemist. After completing high school, Tom Poston attended Bethany College in West Virginia, but did not graduate. While there, he joined Sigma Nu Fraternity. He joined the United States Army Air Forces in 1941. Accepted to officer candidate school and then graduating from flight training, Poston served as a pilot in the European Theater in World War II; his aircraft dropped paratroopers for the Normandy invasion. Poston served in North Africa, Italy, France, and England. After his discharge, he began studying acting in New York City graduating from the American Academy for Dramatic Arts. In the 1950s, Poston gained recognition as a comedic “Man in the Street” (along with his colleagues Louie Nye, Dayton Allen and Don Knotts) on The Steve Allen Show. For these performances, Poston won the 1959 Emmy Award for Best Supporting Actor (Continuing Character) in a Comedy Series. Following that, he appeared frequently on Broadway and as a television game show panelist, including regular appearances on To Tell the Truth and What’s My Line?. Poston’s film career was limited, with appearances in films such as William Castle’s Zotz! (1962), The Old Dark House (1963), Soldier in the Rain (1963), Cold Turkey (1971), The Happy Hooker (1975), Rabbit Test (1978), Up the Academy (1980) and Carbon Copy (1981). However his television career was expansive, covering the better part of five decades, and saw him contributing his comedic talents in virtually every corner of the medium, from made-for-TV movies to variety shows to situation comedies to talk shows and even to voice-overs for cartoons. When Mel Brooks submitted his idea for the television show Get Smart to the ABC network, ABC wanted Poston for the lead role of Maxwell Smart. When ABC passed on the show, the lead went to Don Adams. Poston, however, would make a guest appearance on the show during its run on NBC.

Poston also appeared in Thriller during its second season in 1961. The episode, number six, was entitled “Masquerade” and also starred Elizabeth Montgomery. In the summer of 1968, Poston played the role of the Scarecrow, at The Municipal Opera Association of St. Louis, production of The Wizard of Oz. Lana Cantrell played Dorothy Gale, and Betty Low played the Sorceress of the North, also known as Glinda. Poston was a recurring guest star on The Bob Newhart Show in the 1970s. He later played the role of Franklin Delano Bickley on Mork & Mindy. A longtime friend of Bob Newhart, Poston played George Utley, bumbling country handyman of the Stratford Inn, on Newhart and appeared with Newhart in Cold Turkey as the town drunk, Edgar Stopworth. He was nominated for an Emmy Award three times for Outstanding Supporting Actor in a Comedy Series for his performance on Newhart in 1984, 1986, and 1987. He had a third role with Newhart in the short-lived Bob. Poston also had regular roles on many other television series: Family Matters, Murphy Brown, Home Improvement, Cosby, Malcolm & Eddie, ER, Grace Under Fire, That ’70s Show (as Kitty Forman’s father, Burt Sigurdson), Will & Grace, and guest starred in an episode of The Simpsons as the Capital City Goofball. He also played dentist/jeweler, Art Hibke, on ABC’s Coach, for which he was nominated for an Emmy for Outstanding Guest Actor in a Comedy Series in 1991. He also guest-starred on Home Improvement as a surly airport clerk in Alpena, Michigan when Tim and Al get stuck there during a snowstorm on Christmas Eve.

In 2001, Poston married for the fourth time, to actress Suzanne Pleshette, who played the wife of Newhart’s character Bob Hartley on The Bob Newhart Show. Poston had an affair with Ms. Pleshette in 1959. In that same year, he appeared in The Lone Gunmen episode of “The Cap’n Toby Show”. Poston continued to appear in supporting roles in films, including Krippendorf’s Tribe (1998), The Story of Us (1999), Beethoven’s 5th (2003), and two released in 2004, Christmas with the Kranks and The Princess Diaries 2: Royal Engagement, and on several television programs. In 2005, he played the character “Clown” on the brief-lived NBC series Committed and also guest starred on the ABC series 8 Simple Rules as Rory’s unlawful friend Jake in the episode “Good Moms Gone Wild”. The band They Might Be Giants mentioned Poston as a writer for The New York Times in its song “Critic Intro”. In 2006, Poston guest-starred on an episode of The Suite Life of Zack & Cody entitled “Ah! Wilderness” as Merle, which was his final role. After a brief illness, Poston died of respiratory failure on April 30, 2007, in Los Angeles, California, at the age of 85. Although Poston was not Jewish, he was interred in the Hillside Memorial Park Cemetery (a Jewish cemetery). His widow, the late Suzanne Pleshette, was Jewish.