It broke the news to its employees at a meeting in the cafeteria.
Leamington Mayor John Paterson called the news "a great disappointment."
"This is a day we hoped would never come. A decision of this magnitude could not have been easily by this company," Paterson said. "We deeply regret this news."
John Skocza has worked at Heinz in Leamington since 1978. The plant is approximately 100 years old.
"I’d like to thank Warren Buffet for doing this to Canada. He just crushed us," Skocza said on the way out of the meeting.
Buffett's Berkshire Hathaway and its partner on the deal – 3G Capital — bought Heinz for $28 billion US in February.
Heinz closed three plants in North America on Thursday, including Florence, SC (200 employees); Pocatello, ID (410 employees); and, Leamington, ON in Canada (740 employees).
In a statement, Heinz said it came to its decision "after an extensive review of our company’s North American supply chain footprint, capabilities, and capacity utilization."
"We reached this decision after thoroughly exploring extensive alternatives and options. Heinz fully appreciates and regrets the impact our decision will have on employees and the communities in which these factories are located," Michael Mullen, senior vice president of corporate and government affairs, said in a news release. "We appreciate the many contributions these employees have made to Heinz and we are committed to treating all employees with the utmost respect and dignity."
Heinz plans to offer severance benefits, outplacement services and other support to help affected employees pursue new job opportunities.
"Heinz intends to be generous to its employees," Paterson said.
Ken Hamm grows tomatoes for the Heinz plant in Leamington. He said close to 40 per cent of all field tomatoes grown in Ontario are shipped to Leamington.
He said on average, Heinz would use 225,000 tons of tomatoes annually, at $93-$95 a ton.
“You do the math, that’s a lot of money,” he said.
He said the plant’s closure means he no longer has a buyer. He’ll have to stop growing field tomatoes and start growing soy beans, corn or wheat.
He said the cost of the change is “very substantial” because he has to buy new farm equipment that differs from that used for growing and harvesting tomatoes.
Hamm said his 82-year-old father has been growing tomatoes for Heinz all his life.
CBC News asked Premier Kathleen Wynne, who is also the Minister of Agriculture for comment, but did not receive a response.
In early October, Wynne called on the province's agriculture sector to double its growth by 2020.
The Leamington plant employs 740 full-time workers and an additional number of seasonal workers each year.
In August, H.J. Heinz Co. eliminated 600 office jobs across the U.S. and in Canada, including 350 in Pittsburgh, nearly a third of its operation there.
In Leamington, the Heinz operation is one of the biggest taxpayers and water users in the municipality.