In 1911 Julián established a dry goods store, La Estrella de Oriente (The Star of the Orient). La Estrella de Oriente was an important dry goods store located on Venustiano Carranza where it had merchandise worth more than $100,000 by January 21, 1921, only ten years after the Business was founded. By 1921, he had begun investing in real estate in the flourishing commercial district of Mexico City where Julián would acquire prime real estate at fire sale prices and in Zocalo District during the 1910–17 Mexican Revolution. By 1922, Julián's net worth reached $1,012,258 pesos and was Diversified within various assets including real estate, businesses and various stocks.

In August 1926, Julián Slim married Linda Helú Atta. Linda was born in Parral, Chihuahua, of Lebanese parents who had immigrated to Mexico in the late 19th century. Upon immigrating to Mexico, her parents had founded one of the first Arabic-language magazines for the Lebanese-Mexican community, using a printing press they had brought with them. Julian and Linda had six children: Nour, Alma, Julián, José, Carlos and Linda. Julián senior died in 1953, when Carlos was just 13 years old.

Slim was born on January 28, 1940, in Mexico City, to Julián Slim Haddad (born Khalil Salim Haddad Aglamaz) and Linda Helú Atta, both Maronite Catholics from Lebanon.

From the mid 1960s to the early 1980s, Slim and his growing family lived a modest life, while earnings from Slim's many businesses were re-invested in expansion and more acquisitions. Slim acquired companies with strong returns on capital he believed were undervalued and overhauled their management. He Diversified methodically in numerous industry sectors across the Mexican economy, investing in real estate, then a construction equipment company, and mining companies. The portfolio of Slim companies grew to include a printing company, a tobacco company and Retail stores.

After graduating from college in 1961, Slim began his career as a stock trader in Mexico, often working 14-hour days. In 1965, profits from Slim's private Investments reached US$400,000, allowing him to start the stock brokerage Inversora Bursátil. In addition, he also began laying the financial groundwork for Grupo Carso. In 1965 he also bought Jarritos del Sur. In 1966, worth US$40 million, he founded Inmobiliaria Carso.

Carlos Slim was married to Soumaya Domit from 1967 until her death in 1999. Among her interests were various philanthropic projects, including the creation of a legal framework for organ donation. Slim has six children: Carlos, Marco Antonio, Patrick, Soumaya, Vanessa, and Johanna. Slim's fortune has given rise to a family Business empire and he often acquires shares on behalf of himself and his children. His three older sons serve in key positions in the companies controlled by Slim where most are involved in the day-to-day running of Slim's Business empire. Slim underwent heart surgery in 1999. In high school, Slim's favorite subjects were history, cosmography, and mathematics.

In 1982, the Mexican economy contracted rapidly. As many banks were struggling and foreign Investors were cutting back on investing and scurrying, Slim began investing heavily and bought many flagship companies at depressed valuations. Buying troubled assets at depressed prices to resell them later at an attractive price is a Business strategy that Slim has executed throughout his career.

During the Mexican economic downturn before its recovery in 1985, Slim invested heavily. He bought all or a large percentage of numerous Mexican businesses, including Empresas Frisco, a mining and chemicals company producing silver, gold, copper, lead, and zinc from extracted ores, and also chemical products such as hydrofluoric acid and molybdenum for only $50 million, Industrias Nacobre, a manufacturer of copper products, Reynolds Aluminio, Compania Hulera Euzkadi, Mexico's largest tire maker, Bimex hotels, and majority share of Sanborn Hermanos food retailer, gift shop and restaurant chain. Slim spent US$13 million to buy insurance company Seguros de México in 1984, and later absorbed the company into the firm, Seguros Inbursa. The value of his stake in Seguros eventually became worth US$1.5 billion by 2007, after four spinoffs. He also acquired a 40% and 50% interest in the Mexican arms of British American Tobacco and The Hershey Company, respectively as well as acquiring large blocks of Denny's and Firestone Tires. He moved into financial services as well, buying Seguros de México and creating from it, along with other purchases such as Fianzas La Guardiana and Casa de Bolsa Inbursa, the Grupo Financiero Inbursa. Many of these acquisitions were financed by the revenues and cash flows from Cigatam, a tobacco Business which he bought early in the economic downturn.

Established in 1986 Fundación Carlos Slim Helú sponsors the Museo Soumaya in Mexico City, named after Slim's late wife, Soumaya Domit, opened 2011. It holds 66,000 pieces, including religious relics, contains the world's second-largest collection of Rodin sculptures, including The Kiss, the largest Dalí collection in Latin America, works by Leonardo da Vinci, Pablo Picasso, Pierre-Auguste Renoir, and coins from the viceroys of Spain. The inauguration in 2011 was attended by the President of Mexico, Nobel Prize laureates, Writers and other celebrities.

In 1988 Slim bought the Nacobre group of companies, which trades in copper and aluminum products, along with a chemicals Business, Química Fluor, and others.

Later in 1990, Slim acted in concert with France Télécom and Southwestern Bell Corporation in order to buy the landline telephone company Telmex from the Mexican government, when Mexico began privatizing its national industries. Slim was one of the initial Investors of Telmex, the revenues of the company eventually formed the bulk of Slim's wealth. By 2006, 90 percent of the telephone lines in Mexico were operated by Telmex, and his mobile telephone company, Telcel, which was created out of the Radiomóvil Dipsa company, operated almost 80 percent of all the country's cellphones. By 2012, América Movil, Slim's mobile telephone company, had taken over Telmex and made it into a privately held subsidiary.

In 1991, he acquired Hoteles Calinda (now OSTAR Grupo Hotelero), and in 1993, he increased his stakes in General Tire and Grupo Aluminio to the point where he had a majority interest.

In 1995 Slim established Fundación Telmex, a broad-ranging philanthropic foundation, which as he announced in 2007 had been provided with an asset base of US$4 billion to establish Carso Institutes for Health, Sports and Education. Furthermore, it was to work in support of an initiative of Bill Clinton to aid the people of Latin America. Because Mexican foundations are not required to publish their financial information, it is not possible to confirm Slim's claims of charitable giving through a public source. The foundation has organized Copa Telmex, an amateur Sports tournament, recognized in 2007 and 2008 by Guinness World Records as having the most participants of any such tournament in the world. Together with Fundación Carlos Slim Helú, Telmex announced in 2008 that it was to invest more than US$250 million in Mexican Sports programs, from grass-roots level to Olympic standard. Telmex sponsored the Sauber F1 team for the 2011 season. Telmex donated at least $1 million to the Clinton Foundation.

In 1996, Grupo Carso was split into three companies: Carso Global Telecom, Grupo Carso, and Invercorporación. In the following year, Slim bought the Mexican arm of Sears Roebuck. In July 1997 Grupo Carso agreed in principle to sell Procter & Gamble de México, a subsidiary of The Procter & Gamble Co., a Manufacturing plant in Apizaco and the company's Lypps, Pampys, and other toilet-tissue brands for about US$170 million but kept its tissue-products company Fábricas de Papel Loreto y Peña Pobre.

In 1999, Slim began expanding his Business interests beyond Latin America. Though the bulk of his holdings still remained in Mexico, he began setting his sights towards the United States for overseas Investments.

In 2000, Slim and ex-broadcaster Jacobo Zabludowsky organized the Fundación del Centro Histórico de la Ciudad de México A.C. (Mexico City Historic Center Foundation) to revitalize and rescue Mexico City's historic downtown area to enable more people to live, work and find entertainment there. He has been Chair of the Council for the Restoration of the Historic Downtown of Mexico City since 2001.

Slim became a prominent figure within the American Business scene by 2003 when he began purchasing large stakes in a number of major US Retailers such as Barnes & Noble, OfficeMax, Office Depot, Circuit City, Borders, and CompUSA. Much of reason behind Slim's overseas expansion was due to a running joke in the Mexican Business scene where "there was nothing left to acquire in Mexico". He eyed towards investing the United States where he set up Telmex USA and also acquired a stake in Tracfone, a US cellular telephone company. At the same time, he established Carso Infraestructura y Construcción, S. A. (CICSA) as a construction and engineering company within Grupo Carso. During the same year, Slim had heart surgery and subsequently passed on much of the day-to-day involvement in the businesses to his children and their spouses.

In 2005 Slim invested in Volaris, a Mexican airline and established Impulsora del Desarrollo y el Empleo en América Latina SAB de CV (using the acronym "IDEAL"—roughly translated as "Promoter of Development and Employment in Latin America"), a Mexican construction and civil engineering company primarily engaged in not-for-profit infrastructure development. Since 2006, IDEAL won three infrastructure contracts yet it faces stiff competition from a number of other Mexican and Spanish construction companies. The number of contracts is fewer than its biggest local competitor, Empresas ICA. During the same period, Empresas obtained 18 Mexican projects valued at US$1.09 billion, including airports, toll roads, hospitals and oil platforms. Some of the projects IDEAL has been awarded include the Nezahualcoyotl development, which is a landfill that was acquired for US$150 million by Slim to develop a shopping mall, two schools, a hospital and a park on the site. Other contracts IDEAL has been awarded include a water-treatment plant contract, and a real estate partnership with the Mexican hospital chain Star Médica. IDEAL is also betting on one of the poorest landfills surrounding Mexico City. Slim has also planned to purchase several toll roads offered by the Mexican government that it took over from private companies following the December 1994 currency devaluation. Though speculation that the landfill will take about 12 years to yield a return, the development of a such poor area is revealed to promise handsome Business profits over the years as Grupo Elektra, Mexico's largest consumer electronics retailer, sells 2,000 flat screen televisions a year at its store in the area, making it the third-best-selling outlet. Included in the development, IDEAL will also collect rent from a university, a hospital and a school that will be built around a mall, will have 178 stores, including Inditex's Zara fashion chain and Slim's Grupo Sanborns and the Mexican unit of Sears Holdings. A park in Nezahualcoyotl, the first of its kind will also be constructed. The park will comprise 34 soccer fields, 12 tennis, basketball and volleyball courts, a baseball field and a gym with a swimming pool.

After stating that he had donated US$4 billion of dividends to Fundación Carlos Slim Helú, US$2 billion in 2006, and another US$2 billion in 2010, Slim was ranked fifth in Forbes' World's Biggest Givers in May 2011. Education and health care projects have included $100 million to perform 50,000 cataract surgeries in Peru through the Clinton Initiative, a US$20 million fund to strengthen small and medium-size businesses in Colombia, and a digital education program for youth in Mexico, US$150 million for programs in nutrition and disease prevention in Central America with the bill & Melinda Gates Foundation and the government of Spain, US$50 million to work with the World Wildlife Fund on restoration of six areas for species in Mexico, including the monarch butterfly and US$100 million on education programs for young people through Colombian singer Shakira's Alas Foundation.

On August 8, 2007, Fortune magazine reported that Slim had overtaken Gates as the world's richest person. Slim's estimated fortune soared to US$59 billion, based on the value of his public holdings at the end of July. Gates' net worth was estimated to be at least US$58 billion.

On March 5, 2008, Forbes ranked Slim as the world's second-richest person, behind Warren Buffett and ahead of bill Gates. On March 11, 2009, Forbes ranked Slim as the world's third-richest person, behind Gates and Buffett and ahead of Larry Ellison.

On March 10, 2010, Forbes once again reported that Slim had overtaken Gates as the world's richest person, with a net worth of US$53.5 billion. At the time, Gates and Buffett had a net worth of US$53 billion and US$47 billion respectively. He was the first Mexican to top the list. It was the first time in 16 years that the person on top of the list was not from the United States. It was also the first time the person at the top of the list was from an "emerging economy". Between 2008 and 2010, Slim more than doubled his net worth from $35 to $75 billion.

In 2011, he, along with the President of Mexico, Mexico City mayor, and Mexico City archbishop, inaugurated the first phase of Plaza Mariana close to Basilica de Guadalupe. The complex, whose construction was funded by Slim, includes an evangelization center, museum, columbarium, health center, and market.

In December 2012, according to the Bloomberg Billionaires Index, Carlos Slim Helú remained the world's richest person with an estimated net worth of US$75.5 billion.

On May 16, 2013, Bloomberg L.P. ranked Slim the second-richest person in the world, after bill Gates.

Slim's growing fortune has been a subject of controversy, because it has been amassed in a developing country where average per capita income does not surpass US$14,500 a year, and nearly 17% of the population lives in poverty. Critics claim that Slim is a monopolist, pointing to Telmex's control of 90% of the Mexican landline telephone market. Slim's wealth is the equivalent of roughly 5% of Mexico's annual economic output. Telmex, of which 49.1% is owned by Slim and his family, charges among the highest usage fees in the world, according to the Organisation for Economic Co-operation and Development. The average Mexican spends 1.50 pesos per day on Slim's goods and services for a total of roughly US$140 million a day and the Federal Telecommunications Institute, a new Mexican government anti-monopoly watchdog said in April 2014 that Slim's Telecom businesses are monopolies. Slim's Business presence in Mexico alone is so broad that many Mexicans find it appropriate to call the country "Slimlandia" as it is almost impossible to go a day in Mexico without contributing to Slim's wealth.

Slim was criticized by the Dutch minister of economic affairs, Henk Kamp, in 2013 for attempting to expand his Telecom empire beyond the Americas by América Móvil's buy-out offer to KPN, a Dutch landline and mobile telecommunications company privatized in the 1990s, by stating "an acquisition of KPN by a 'foreign company' could have consequences for the Netherland's national security". Two years after Slim's failed bid to take over the company mainly due to political intervention and Slim's paucity of interest in purchasing the company, Slim's America Movil SAB began offering 2.25 billion euros of bonds that can be converted into shares of Royal KPN NV. America Movil now controls a 21.1 percent stake of KPN with a market value of 3.1 billion euros as of May 20, 2015. Slim has been slowly decreasing his holdings since he was forced to withdraw a 7.2 billion euro bid for the Dutch phone line carrier in 2013 after negotiations broke down and KPN's preference share foundation blocked the takeover attempt.

As of December 2016 his net worth was US$48.1 billion.

In 2017, his net worth was reported to be $54.5 billion.

Julian’s Business ventures became the source of considerable wealth for himself and his family. As a result of financial prosperity of these ventures, Julian soon became a prominent and wealthy businessman, where he was able to make Investments during bad economic cycles due to Mexico's frequent economic downturns. Julián was known for his Business savvy, strong work ethic, and commitment to traditional Lebanese moral values. Having a deep understanding of Business that was considered ahead of his time, one of Julián's many pioneering Business concepts was an efficient Business as one that sold large volumes at smaller margins, and with payment facilities, factors that are prevalent in many large discount stores today.