The morning of Oct. 5, 1970, James “Jasper” Cross, a senior British diplomat posted in Canada, was readying himself for the day, dressed only in his shirt and undergarments, when armed intruders entered his home in an elegant neighborhood of Montreal. Under the gaze of his wife and their Dalmatian, the gunmen ordered Mr. Cross to the ground, handcuffed him and, after allowing him to dress, spirited him away in a taxi.

The assailants belonged to the Front de Libération du Québec, or FLQ, a militant group that called for the independence of their Francophone province in eastern Canada. Mr. Cross’s kidnapping — followed shortly thereafter by the capture and then death of Pierre Laporte, Quebec’s minister of labor — precipitated what Canadians call the October Crisis and made headlines around the world.

Mr. Cross, who endured 59 days of captivity before his release was secured through high-wire negotiations with the separatists, died Jan. 6 at his home in Seaford, in East Sussex, England. He was 99 and had covid-19, the disease caused by the novel coronavirus, said his son-in-law, John Stringer.

The October Crisis, when soldiers policed the streets of Montreal and hundreds were arrested after Prime Minister Pierre Trudeau invoked Canada’s War Measures Act, represented a grim milestone in the long history of tensions surrounding the political status of Quebec.

Quebec nationalists, many of whom at the time harbored economic grievances against the English-speaking majority of Canada, argued that the province was culturally distinct from the rest of the country and therefore should be politically independent as well.

Mr. Cross, who was born in Ireland and thus was well-acquainted with violent political passions, arrived in Montreal as trade commissioner in 1968. The FLQ, which had carried out a string of bombings, executed its most notorious attack the next year, detonating a bomb at the Montreal Stock Exchange that injured dozens.

When members of the FLQ kidnapped Mr. Cross, they demanded $500,000 in gold, the publication of a manifesto and the release of compatriots they described as “political prisoners.” They initially set a deadline of 48 hours, prompting Mr. Cross, he later said in an oral history with the University of Cambridge, to conclude that “I must compose myself for death.”

After the manifesto aired on the radio, the militants agreed to extend the deadline, according to an obituary of Mr. Cross published by the Canadian Broadcasting Corp.

During his captivity, Mr. Cross was handcuffed and fed a quantity of food so insufficient, he said, that he lost 22 pounds. He was permitted to watch television and read — in addition to political tracts, his captors furnished him with French translations of the novels of Agatha Christie — and to pass time with single-player card games.

A photo released weeks into his ordeal, showing Mr. Cross playing cards while seated on what appeared to be a container of explosives, became, in the description of the CBC, one of “the most iconic photos in Canadian history, representative of the moment when Quebec appeared to be teetering on the brink of insurrection.”

Mr. Cross also recalled keeping himself company by trying to excavate distant memories.

“You have an awful lot of time to reflect, as you’re sitting there hour after hour, day after day,” he told the Montreal Gazette in 2004. “One of the techniques I had was to try to relive a walk to school I did across fields when I was about 6 or 7.”

“In the beginning,” he continued, “I could recollect a few trees and things. By the end, I could probably recollect every blade of grass. I did the same things with holidays we took, books I’d read, films I’d seen — anything to occupy one’s mind.”

Mr. Cross’s captors ultimately released him on the condition that they be permitted to flee to Cuba. (They later returned to Canada and served prison sentences.)

“Seeing daylight again,” Mr. Cross told the Gazette, “was the most beautiful sensation, to see sunlight which I had not seen for eight weeks.”

James Richard Cross was born Sept. 29, 1921, in Nenagh, a town in County Tipperary. His mother, whose older child suffered from intellectual disabilities and was institutionalized, died in childbirth. Mr. Cross grew up on his father’s farm and later with relatives.

He studied economics at Trinity College in Dublin, then joined the British army and was stationed in what was then the British Mandate of Palestine. According to the Gazette, he survived a Zionist attack on the King David Hotel in Jerusalem in 1946 that killed more than 90 people.

He subsequently joined the British civil service — later joking that he “thought it would be a nice quiet occupation” — and served in India and Malaysia before he was posted in Canada. After his kidnapping, he returned to England and spent the rest of his diplomatic career in London.

In 1995, Quebecois voters rejected a referendum proposing separation from Canada, but only by a margin of approximately 1 percentage point.

Mr. Cross’s wife, the former Barbara Dagg, died in 2018 after 73 years of marriage, and their only child, Susan Cross, died in 2015. Mr. Cross had no immediate survivors.

Mr. Cross “never condoned the kidnappers,” his son-in-law said, but neither did he allow “what they did to embitter him or eat into his enjoyment of life thereafter.”

“He never forgave them,” Stringer observed, “but never dwelled on the matter.”