Slim was born in 1940 into a family of Lebanese Christian immigrants to Mexico, where his father made a fortune in real estate during the Mexican Revolution of 1910–20. Slim received a degree in engineering from the National Autonomous University of Mexico, and by the mid-1960s he was investing in a variety of businesses that became the foundation for Grupo Carso. He attained billionaire status on the heels of the economic crash of 1982, when the Mexican government, defaulting on foreign debts in light of a devalued peso, began nationalizing banks and scaring away business investors. Having purchased at bargain prices controlling interests in a variety of companies, Slim managed them so effectively that within the span of a decade their sum value had soared.

For more than a dozen years, Slim's key holding and the anchor to his success had been his ownership of the former national telephone monopoly, Teléfonos de México (Telmex), which had allowed him to broaden his investment portfolio into American technology and telecommunications firms such as Prodigy Inc. and SBC Communications Inc. Grupo Carso also held extensive interests in numerous Mexican companies.

By the late 1980s Slim had forged close ties with President Carlos Salinas de Gortari (Salinas de Gortari, Carlos) and the ruling Institutional Revolutionary Party. In 1990 the Gortari administration privatized Telmex, and Slim, along with SBC and France Télécom, made the $1.76 billion purchase. Slim later won management control of Telmex, alienating France Télécom but keeping close relations with SBC. His tight control of Telmex upset his competitors, as well as some consumers critical of the communications giant. By 2003, however, he had turned over much of his business operations to his three sons.

Slim, a noted art collector and philanthropist, was prominent in the revitalization of the historic centre of Mexico City and supported anticrime efforts there. In late 2002 he led a group that invited former New York City mayor Rudolph Giuliani to assist in combating crime in Mexico's capital. Having founded the Foundation for the Historic Centre of Mexico City in 2000, Slim received the Hadrian Award from the World Monuments Fund in 2004 for his efforts in preserving culturally significant buildings in Mexico City.