Romero was born Cesar Julio Romero, Jr. in New York City on February 15, 1907, the son of Cesar Julio Romero, Sr. and Maria Mantilla (daughter of Cuban national hero José Martí). His father was an Italian-born importer-exporter of sugar refining machinery, and his mother was a Cuban concert singer. He grew up in Bradley Beach, New Jersey, and was educated at Bradley Beach Elementary School, Asbury Park High School, the Collegiate School, and the Riverdale Country Day School. However, that lifestyle 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 United States 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, Romero 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 rating 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 6 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. Romero had a lead role as the Pathan rebel leader, Khoda Khan, in John Ford’s 1937 British Raj-era actioner Wee Willie Winkie alongside Shirley Temple. He also appeared in a comic turn as a subversive opponent to Frank Sinatra and his crew in Ocean’s 11.  Romero was also a romantic if aggressive leading man in films such as Allan Dwan’s 15 Maiden Lane (1936) opposite Claire Trevor, in which he spins Trevor around in a dance sequence, and played the key role of the Doc Holliday character (with name changed to “Doc Halliday”) in Dwan’s Wyatt Earp saga Frontier Marshal three years later.  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 many television credits, Romero appeared several times on The Martha Raye Show in the mid 1950s. He portrayed Don Diego de la Vega’s uncle in a number of Season 2 Zorro episodes.  In 1958, he guest-starred as Ramon Valdez, a South American businessman, who excels at dancing 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 the character Don Carlos, a card shark on the episode, “The Honorable Don Charlie Story,” of NBC’s Wagon Train.  On January 16, 1958, Romero appeared on The Ford Show, Starring Tennessee Ernie Ford.  In 1959, Romero was cast as Joaquin in the episode “Caballero” from The Texan.  In 1960, he was cast as Ricky Valenti in “Crime of Passion” from Pete and Gladys.  In 1965, Romero played the head of THRUSH in France in “The Never Never Affair” from The Man from U.N.C.L.E..

From 1966 to 1968, Romero portrayed the Joker in Batman. He refused to shave his mustache for the role and so, the Joker’s white face makeup was simply smeared over it when playing the supervillain throughout the series’ run and in the 1966 film. However, Romero’s facial hair can still be seen in some shots. He also appeared in an episode of Daniel Boone as 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 stated that his grandfather on his mother’s side was Cuban poet and patriot José Martí.  Romero never married but made frequent appearances at Hollywood events, escorting actresses such as Joan Crawford, Linda Darnell, Barbara Stanwyck, Lucille Ball, Ann Sheridan, Jane Wyman, and Ginger Rogers; he was almost always described in interviews and articles as a “confirmed bachelor”. Many Hollywood historians have speculated that Romero was a closeted gay man. Marlene Dietrich was quoted as observing that Romero was, “the undisputed queen of homosexuals. I don’t think there was a gay actor in all Hollywood who hadn’t been there.” In 1996, biographer Boze Hadleigh revealed Romero confessed he’d had a sexual encounter with friend Desi Arnaz.  Romero was a registered Republican and in October 1960 he appeared in the Nixon-Lodge Bumper Sticker Motorcade Campaign.  On January 1, 1994, Romero died of bronchitis and pneumonia in Santa Monica, California, at age 86. His body was cremated and the ashes were interred at Inglewood Park Cemetery in Inglewood, California.