The magazine estimates that Mr Slim, whose business interests range from telecommunications to construction, is worth $73bn (£49bn).
He is followed by Microsoft founder Bill Gates on $67bn.
Famed investor Warren Buffett has dropped back to fourth place, with third spot going to Amancio Ortega, the founder of the Zara fashion chain.
Record profits, and a resulting surge in the share price of the holding company Inditex that he owns, has propelled Mr Ortega from fifth place, leapfrogging Mr Buffett.
In contrast, the “Sage of Omaha” admitted last week that his Berkshire Hathaway investment company had a “sub-par” year in 2012, underperforming the S&P 500 index of US share prices for only the ninth time in its 48-year history.
Mr Buffett, 82, was one of seven over-70s in the 10 ten wealthiest people on the list, with the average age of the top 10 reaching 74 this year.
Besides Mr Ortega, another fast riser has been Larry Ellison, the Oracle tycoon, whose software firm has risen 20% on the stock market over the last year.
There have been mixed fortunes in the French luxuries sector. Bernard Arnault, the man behind Louis Vuitton Moet Hennessey, saw his stock fall last year, dragging him down from fourth spot to 10th.
That put him one place behind 90-year-old L’Oreal heiress Liliane Bettencourt, whose estimated $30bn fortune has been put under custody of her daughter after a long legal battle.
The two Koch Brothers, who own a giant US industrial conglomerate and have helped bankroll the radical right-wing Tea Party movement, came in joint sixth place, with $34bn a piece.
Li Ka-Shing has held onto the ninth position and remains the highest-placed Asian, with $31bn. The Hong Kong-based owner of Hutchison Whampoa and Cheung Kong Holdings is the world’s biggest operator of container terminals.
Reprinted from BBCnews