It is the first time since 1994 that someone from the US has not been ranked first.
Mr Slim is famed for his lack of ostentation and aggressive investments during crises.
The 70-year-old took the top spot for the first time pushing the Microsoft founder out as he rose from third place on the success of America Movil, Latin America's biggest mobile phone operator.
His fortune was estimated at $53.5bn, according to the new Forbes ranking, which was published yesterday.
'These numbers come from the price of the stock of the companies so we're happy that investors are confident about Mexico, confident about Latin America and confident about our companies,' Arturo Elias Ayub, spokesman for Slim's Telmex group, told AFP.
Mr Slim's fortune rose $18.5bn in 12 months, and shares of America Movil, of which he owns a $23bn stake, were up 35% in a year, according to Forbes.
Mr Slim learned his business acumen at an early age; his father, a Lebanese immigrant, gave each of his children a savings book for managing their income and expenses.
He studied civil engineering and later built up the telephone monopoly Telmex after acquiring it from the government in 1990.
The softly-spoken billionaire last month received authorisation to merge three of his telecommunications companies to form a regional giant, with 250 million customers in 18 countries.
His business empire is ever-present across Mexico, including department stores, building companies and the Inbursa financial group.
Amid the financial crisis of 2008, he continued his trademark behavior of buying up struggling businesses, for which he first became famous during the Latin American economic crisis of the early 1980s.
'Instead of stopping investing, he invests more when a crisis comes and the results have always been good,' Mr Ayub said.
In 2008, Mr Slim bought a minority stake in The New York Times as the stock sunk.
He has recently made investments in telecommunications across the Americas, and also in infrastructure, including water and electricity companies, Mr Ayub said.
Mr Slim, a widower with six children, has handed over the daily operations of his companies to his three sons and business partners and is a well-known public figure in Mexico.
His is also a baseball fanatic, and known for his philanthropy although not on the scale of Bill Gates.
Through two foundations he has invested some $10bn in health, education, justice and sports projects in Mexico and Latin America.
Forbes Top 10 Rich List
The top 10 richest people in the world have a combined worth of $342b in 2010, up from $254bn a year ago. Here are the 20 wealthiest, as announced yesterday by Forbes magazine in its annual list of the world's billionaires:
1. Carlos Slim Helu and family. Mexico. Telecoms. $53.5bn
2. Bill Gates. US. Microsoft. $53bn
3. Warren Buffett. US. Berkshire Hathaway. $47bn
4. Mukesh Ambani. India. Oil, gas. $29bn
5. Lakshmi Mittal. India. Steel. $28.7bn
6. Lawrence Ellison. US. Oracle. $28bn
7. Bernard Arnault. France. LVMH luxury goods. $27.5bn
8. Eike Batista. Brazil. Mining, oil. $27bn
9. Amancio Ortega. Spain. Clothing retail. $25bn
10. Karl Albrecht. Germany. Supermarkets. $23.5bn
11. Ingvar Kamprad and family. Sweden. Ikea. $23bn
12. Christy Walton and family. US. Walmart. $22.5bn
13. Stefan Persson. Sweden. Hennes and Mauritz. $22.4bn
14. Li Ka-shing. Hong Kong. Diversified. $21bn
15. Jim Walton. US. Walmart. $20.7bn
16. Alice Walton. US. Walmart. $20.6bn
17. Liliane Bettencourt. France. L'Oreal. $20bn
18. S. Robson Walton. Walmart. $19.8bn
19. Prince Alwaleed Bin Talal Alsaud. Saudi Arabia. Investments. $19.4bn
20. David Thomson and family. Canada. Inheritance. $19bn
