Colacello began his writing career around 1969, when he began publishing film reviews in the Village Voice weekly. As a graduate student in the Film department at Columbia University in New York, his first publications doubled as his class essays and homework assignments. In 1970, Colacello wrote a review of Andy Warhol's film Trash, which he hailed as a "great Roman Catholic masterpiece". This review garnered the attention of Warhol, and Paul Morrissey, the director of many of Warhol's films, who approached Colacello to write for Interview magazine, a new art/film/fashion magazine Warhol had recently began to publish.Colacello was made editor of Interview within six months and, for the next 12 years, remained directly involved in all aspects of life and business at The Factory, Warhol's infamous studio, as he developed the magazine into one of the best-known lifestyle magazines of the time. As Colacello himself writes in Andy Warhol Close up (1990), Warhol suggested Colacello change his name to Bob Cola, in order to sound more "pop."

After his tenure with Interview, Colacello began writing for Vanity Fair magazine, and has been a regular contributor since, writing extended profiles on a wide range of public personalities, including Prince Charles and Camilla Parker Bowles, Balthus, Rudolf Nureyev, Liza Minnelli, Estée Lauder, Doris Duke, and Naomi Campbell. Colacello has also established himself as one of the most prolific biographical writers in the United States. He is the author of the highly praised Their Path to the White House, 1911-1980, about the social and political rise of Ronald Reagan and his wife Nancy Reagan. His memoir of working with Andy Warhol in the 1970s and early 1980s, titled Andy Warhol Close Up, was called the "best-written and the most killingly observed" book on the subject by the New York Times .