Warren Buffett's Berkshire Hathaway Inc
Although the investment represents less than 1 percent of Exxon's shares, it signals Buffett's approval of one of the world's largest and most profitable companies.
Exxon shares rose 0.7 percent in after-hours trading following the disclosure.
"He likes buying big, established global brand names, and Exxon is a good flight-to-quality stock," said Fadel Gheit, senior oil analyst at Oppenheimer & Co, referring to Buffett. Gheit has a "perform" rating on Exxon.
"The stock has also lagged the market in the last three and five years. That makes it a typical Warren Buffett holding."
The Exxon purchase was disclosed in a U.S. Securities and Exchange Commission filing that detailed $92 billion of Berkshire equity investments, mostly listed on U.S. exchanges, as of Sept 30.
Exxon spokesman Alan Jeffers said: "We appreciate the confidence investors have in ExxonMobil when they decide to invest in the company."
Berkshire already has energy and utilities businesses, including MidAmerican Energy, which is spending $5.6 billion to buy Nevada utility NV Energy Inc .
Debbie Bosanek, an assistant to Buffett, did not immediately respond to a request for comment.
U.S. regulators require large investors to disclose their stock holdings every quarter, and the disclosures can offer a window into their strategies for buying and selling stocks.
Buffett sometimes gets SEC permission to delay disclosures so that he can buy stocks without having investors copy him. He had done this for the second quarter of this year, and revealed on Thursday that Exxon purchases took place during that period.
The second-richest American typically makes Berkshire's largest investments, while other investments are made by portfolio managers Todd Combs and Ted Weschler, who oversee several billion dollars each. Berkshire's quarterly filings do not reveal who buys or sells what.
Among the other disclosed changes was a 44 percent drop from the prior quarter in its share stake in another oil company, ConocoPhillips .
In Thursday trading, Berkshire's Class A shares closed up 0.7 percent at $173,320, and its Class B shares rose 0.8 percent to $115.69.
(Editing by Gary Hill and Ken Wills)