Showalter was born in Caldwell, Kansas, the son of Elma Roxanna (Dodson) Showalter (1889–1953), a music Teacher, and Ira Edward Showalter (1887–1953), who worked in the oil industry and was also a banker and farmer. He developed a Desire for acting as a toddler while accompanying his mother to local theatres where she played piano for silent movies.

By the late 1930s, Showalter had multiple stage roles under his belt, including acting in productions of the Pasadena Playhouse. He soon made his Broadway to debut in Knights of Song. Showalter also appeared in the traveling musical This Is the Army for two years and in other notable Broadway productions like Make Mine Manhattan and The Grass Harp. His most memorable stage role was as Horace Vandergelder in the Broadway hit show, Hello Dolly!. Showalter performed the role more than 3,000 times opposite Carol Channing, Betty Grable and Ethel Merman.

In the late 1940s, Showalter was signed to 20th Century Fox as a featured contract player. His name was changed by Fox's founder, Darryl F. Zanuck to the more "bankable" Casey Adams.

He made his feature film debut in Always Leave Them Laughing (1949). He first appeared on live television in the short-lived musical variety series The Swift Show (1948–49), also known as The Lanny Ross Show.

In the 1950s, Showalter took a hiatus from his work in Hollywood, returning to Caldwell, Kansas, to care for his 15-year-old sister who was orphaned by the death of their parents in an automobile accident. Their deaths followed the death of Showalter's brother, Robert, in a car wreck two years earlier. After "a couple of years" he returned to Hollywood and resumed his career.

Showalter's second feature film was the biopic With a Song in My Heart (1952), where he had a small role as a vaudeville performer. In the film, Showalter, along with David Wayne, sang the song "Hoe that Corn", which he also wrote. He appeared in Niagara (1953) alongside Marilyn Monroe and Joseph Cotten. He made a cameo as a Life magazine Photographer in another Monroe movie, Bus Stop, in 1956.

In 1956, Showalter (as Casey Adams) recorded an album of his own music, Casey Adams Plays and Sings Max Showalter Songs (Foremost Records FML-1004). He was one of the artists featured on The Secret Garden, a 1988 CBS Special Products album containing performances of music from the musical of that title that "has played the repertory circuit in England."

In the 1960s, Showalter reclaimed his original name and continued to land roles in such big-budget films as Elmer Gantry (1960), The Music Man (1962), and How to Murder Your Wife (1965). He worked through the 1960s and 1970s. He made six appearances on Perry Mason, including the role of murderer Carl Reynolds in the 1958 episode, "The Case of the Curious Bride," murder victim Burt Stokes in "The Case of the Wandering Widow" in 1960, and murderer Talbot Sparr in the 1964 episode, "The Case of the Ugly Duckling." He made appearances in other television series like The Twilight Zone (the iconic episode "It's a Good Life"), The Lucy Show, Gunsmoke, The Many Loves of Dobie Gillis, Bewitched, Dr. Kildare, Surfside 6, The Doris Day Show, Kojak, Police Story, The Bob Newhart Show, as well as in cult films, Lord Love a Duck, The Anderson Tapes and Sgt. Pepper's Lonely Hearts Club Band. In the 1979 film 10, he famously played a pastor whose hobby was writing (bad) songs. He was also a regular cast member in the short-lived 1980 TV series, The Stockard Channing Show. Showalter made his last onscreen appearance in the John Hughes film Sixteen Candles (1984).

Show Business columnist Hedda Hopper reported in a 1963 newspaper column that Showalter had sold 139 paintings and would have his first one-man show.

In 1984, Showalter retired from acting and moved to an 18th-century farmhouse in Chester, Connecticut, near the area where he acted in the film, It Happened to Jane (1959). Showalter became involved in local musical theatre, including the Ivoryton Playhouse, and went on to produce, direct, write, and narrate the Christmas musical Touch of a Child. Also, Showalter spent much of his free time painting oil miniatures.

On July 30, 2000, Max Showalter died of cancer in Middletown, Connecticut. He was 83 years old.