Apple Inc. surpassed Microsoft Corp. Monday as the largest U.S. company ever, measured by stock-market value.

Apple hit the new milestone—$623.52 billion—at a time when its influence on the economy, on the stock market and on popular culture rivals that of some of the most powerful companies in U.S. history: General Motors Co., GM -0.64% whose Corvette and Impala typified a confident postwar manufacturing giant; Microsoft, whose technology heralded the arrival of the personal computer and the early Internet age; and International Business Machines Corp., IBM -0.36% whose buttoned-down rigor inspired rivals to reach for greatness.

“It is one of those iconic companies,” says Richard Sylla, professor of financial history at New York University’s Stern School of Business. “When I think about these companies, their products were used by all kinds of people and their leaders were considered geniuses.”

Apple’s stock-market valuation at Monday’s close of business was greater, in nominal terms, although not in inflation-adjusted terms, than Microsoft’s closing high of $616.34 billion in December 1999. It left Apple’s value more than $200 billion ahead of that of its nearest U.S. rival, Exxon Mobil, which stood at $405.97 billion, according to FactSet. That gap is almost as large as the market value of IBM.

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