Cesar Romero was born in New York City, the son of Maria Mantilla and Cesar Julio Romero, Sr. His father, a native of Italy, was an importer-exporter of sugar refining machinery, and his mother was a Cuban concert singer. That lifestyle, however, changed dramatically when his parents lost their sugar import business and suffered losses in the Stock Market Crash of 1929. Romero's Hollywood earnings allowed him to support his large family, all of whom followed him to the American West Coast years later. Romero lived on and off with various family members, especially his sister, for the rest of his life.
In October 1942, he voluntarily enlisted in the U.S. Coast Guard and served in the Pacific Theater of Operations. He reported aboard the Coast Guard-manned assault transport USS Cavalier in November, 1943. According to a press release from the period he saw action during the invasions of Tinian and Saipan. The same article mentioned that he preferred to be a regular part of the crew and was eventually promoted to the rate of Chief Boatswain's Mate.
Romero played "Latin lovers" in films from the 1930s until the 1950s, usually in supporting roles. He starred as The Cisco Kid in six westerns made between 1939 and 1941. Romero danced and performed comedy in the 20th Century Fox films he starred in opposite Carmen Miranda and Betty Grable, such as Week-End in Havana and Springtime in the Rockies, in the 1940s. He also played a minor role as Sinjin, a piano player in Glenn Miller's band, in the 1942 20th Century Fox musical, Orchestra Wives.
In The Thin Man (1934), Romero played a villainous supporting role opposite the film's main star William Powell. Many of Romero's films from this early period saw him cast in small character parts, such as Italian gangsters and East Indian princes. He also appeared in a comic turn as a subversive opponent to Frank Sinatra and his crew in Ocean's 11.
20th Century Fox, along with mogul Darryl Zanuck, personally selected Romero to co-star with Tyrone Power in the Technicolor historical epic Captain from Castile (1947), directed by Henry King. While Power played a fictionalized character, Romero played Hernán Cortés, a historical conquistador in Spain's conquest of the Americas.
Among his many television credits, Romero appeared several times on NBC's The Martha Raye Show in the middle 1950s. He played the role of Don Diego de la Vega's uncle in a number of Season Two Zorro episodes on ABC.
In 1958, he guest starred as Ramon Valdez, a South American businessman, who excels at doing the Cha-Cha with Barbara Eden in her syndicated romantic comedy, How to Marry a Millionaire in the episode entitled "The Big Order". He performed the mambo with Gisele MacKenzie on her NBC variety show, The Gisele MacKenzie Show. He guest-starred in 1957 on CBS's The Lucy-Desi Comedy Hour on the first episode of the seventh season ("Lucy Takes a Cruise to Havana"). He also played a card shark on the episode, "The Honorable Don Charlie Story," of NBC's Wagon Train.
On January 16, 1958, Romero appeared on NBC's The Ford Show, Starring Tennessee Ernie Ford.
In 1959, Romero was cast as the title character, Joaquin, in the episode "Caballero" of the CBS western series, The Texan, starring Rory Calhoun. Veteran character actor Whit Bissell appears in this episode as Shep Crawford.
In 1960, he was cast as Ricky Valenti in the episode "Crime of Passion" of the CBS sitcom, Pete and Gladys, starring Harry Morgan and Cara Williams.
In 1965, Romero played the head of THRUSH in France in the episode "The Never Never Affair" of NBC's The Man from U.N.C.L.E..
From 1966 to 1968, Romero played the Joker in ABC's television series, Batman. He refused to shave his mustache and so the Joker's white face makeup was simply smeared over it when playing the supervillain throughout the series' run, and in the spin-off 1966 film. However, Romero's facial hair can still be seen in some shots. Romero also appeared in an episode of Daniel Boone as Spanish army captain, Esteban de Vaca in 1966.
In the 1970s, Romero portrayed the absent father of the Freddie Prinze character Chico Rodriguez in Chico and the Man, and later Peter Stavros in the television series Falcon Crest (1985–1987). Among Romero's guest star work in the 1970s was a recurring role on the western comedy Alias Smith and Jones, starring Pete Duel and Ben Murphy. Romero played Señor Armendariz, a Mexican rancher feuding with Patrick McCreedy (Burl Ives), the owner of a ranch on the opposite side of the border. He appeared in three episodes. He also appeared as Count Dracula on Rod Serling's Night Gallery, and guest-starred in an episode of Bewitched.
Apart from these television roles, Romero appeared as A.J. Arno, a small-time criminal who continually opposes Dexter Riley (played by Kurt Russell) and his schoolmates of Medfield College in a series of films by Walt Disney Productions in the 1970s. He also appeared in a sixth-season episode of The Golden Girls, where he played a suitor named Tony Delvecchio for Sophia who disappoints her when she tells him "I love you" after a night of passion and he doesn't return the sentiment.
Romero said his grandfather on his mother's side was Cuban poet and patriot José Martí.
He never married, but made regular appearances on the Hollywood social circuit in the company of attractive actresses; however, he was almost always described in interviews and articles as a "confirmed bachelor".
Romero died on New Year's Day 1994 from bronchitis and pneumonia. His ashes were interred at Inglewood Park Cemetery in Inglewood, California.