On the news, Kraft shares surged more than 30 percent at the open on heavy volume—trading at all-time highs dating back to its 2012 spinoff from then Kraft Foods Inc., which was renamed Mondelēz International.
"The short term doesn't make much difference to us, because we will be in this stock forever," Buffett said. "This is a business with us. It's not really a stock. It's a company that we'll own 26 and a fraction percent of."
"It's where the new Kraft Heinz Co. is 10, 20, 50 years from now that counts to Berkshire," he added. "These are brands I liked 30-plus years ago, and I like them today. And I think I'll like them 30 years from now."
Besides the Kraft brand, Kraft Foods is behind other household names such as Capri Sun, Jell-O, Kool-Aid, Lunchables, Maxwell House, Oscar Mayer, Philadelphia cream cheese, Planters and Velveeta.
In addition to ketchup and sauces, Heinz owns Ore-Ida potato products, Weight Watchers Smart Ones entrees, and T.G.I. Friday's snacks.
Read MoreBuffett bonanza? Heinz and Kraft to merge
Buffett's involvement in Heinz started in 2013, when Berkshire and Brazilian private-equity firm 3G Capital bought it for $23 billion and took it private.
"We put in $4.25 billion to buy common stock of Heinz initially," he said "[Now] we're putting in a little over $5.2 billion, ... so we'll have $9.5 billion roughly in the common stock. And we'll own 320-odd million shares of the new company."
Under the terms of the deal, Heinz will return to the public market with a 51 percent ownership of the new company. Current holders of Kraft stock will own 49 percent.
"There are presently about 600 million Kraft shares outstanding. After the deal, there will be roughly 1.2 billion [shares] or so," Buffett said.
Kraft shareholders will receive stock in the combined company, and a special cash dividend of $16.50 per share, financed by a $10 billion investment from 3G and Berkshire.
Buffett said there will be six directors on the board of the new company from the Heinz side—three from Berkshire and three from 3G—and five directors from the former Kraft operation.
"I like the management a whole lot," he said—putting faith in Heinz CEO Bernardo Hees who will run the new company.