Forbes said its real-time ranking of the world's billionaires now estimates Buffett's wealth at $74.4 billion, about $1.5 billion more than Carlos Slim 's $72.9 billion.
Class A shares of Buffett's Berkshire Hathaway closed at a record $227,800 after hitting an all-time intraday high of $229,374 in Monday's trading. They're up 28 percent for the year, trouncing the S&P's 11.5 percent gain, excluding dividends.
In March, Forbes estimated Buffett's fortune at $58.2 billion. He was No. 4 on the list at that time, behind Mexico's Slim with $72 billion and Spanish fashion executive Amancio Ortega with $64 billion.
Forbes notes that Slim's net worth has dropped $2 billion since its peak in early September as shares of his America Movil company slip on expectations it will have to sell assets to keep its market share below 50 percent, as required by new antitrust regulations in Mexico.