Buffett did not mention the divorce as a reason to resign, while noting he has given up all directorships outside Berkshire, reducing his workload.
The Gates Foundation did not immediately respond to a request for comment.
Buffett donated $4.1 billion of Berkshire shares to the Gates Foundation and four family charities on Wednesday, part of his 2006 pledge to donate about 99% of his net worth to philanthropy.
He has since donated more than $41.5 billion of Berkshire shares, a sum now equal to about $100 billion because the stock price has risen.
The Gates Foundation received four-fifths of the donations. Founded in 2000, it focuses on combating poverty, disease and inequity, spending $54.8 billion in its first two decades.
Arguably the world's most famous investor, Buffett has since 1965 built Omaha, Nebraska-based Berkshire into a more than $600 billion conglomerate, owning businesses such as the BNSF railroad and Geico auto insurance, and stocks such as Apple Inc.
"Over many decades I have accumulated an almost incomprehensible sum simply by doing what I love to do," Buffett said in a statement. "Society has a use for my money: I don't."
Wednesday's donations also go to the Susan Thompson Buffett Foundation, named for Buffett's late first wife, and charities run by Buffett's children Howard, Susan and Peter: the Howard G. Buffett Foundation, the Sherwood Foundation and the Novo Foundation.
Had Buffett not made his donations, his fortune would roughly equal that of Amazon.com Inc founder Jeff Bezos, the world's richest person according to Forbes magazine.
Bill Gates and Buffett also pioneered "The Giving Pledge," where more than 200 people like Michael Bloomberg, Larry Ellison, Carl Icahn, Elon Musk, Mark Zuckerberg and Saudi Prince Alwaleed bin Tall committed at least half their fortunes to philanthropy.
Buffett's statement also addressed recent criticism after a ProPublica investigation found that he and other wealthy people paid low taxes relative to their fortunes.
He said his donations have resulted in only about 40 cents of tax savings per $1,000 given.
"Nevertheless, tax deductions are important to many-- particularly to the super-rich--who give large amounts of cash or securities to philanthropy," he said.
Buffett also used references to American football to assure fans he has no plans to step away, including from Berkshire.
"These remarks are no swan song," he said. "I still relish being on the field and carrying the ball. But I'm clearly playing in a game that, for me, has moved past the fourth quarter into overtime."
(Reporting by Jonathan Stempel in New York and Eva Mathews in Bengaluru; Editing by Anil D'Silva and Nick Zieminski) ((Eva.Mathews@thomsonreuters.com;))