A socialist and trade unionist who began his career at France’s central bank, Mr. Delors worked as an economics lecturer and as a senior official in ideologically divergent French governments.

Under President François Mitterrand, Mr. Delors served as economics and finance minister from 1981 to 1983 and as economics, finance and budget minister in 1983 and 1984. He was considered a moderate in the country’s first socialist government since 1956. In the face of growing inflation, he pushed for what some socialists called “austerity with a human face,” putting the brakes on spending and introducing new taxes.

In 1985, when European unity still appeared tenuous, Mr. Delors was appointed president of the Brussels-based European Commission with Mitterrand’s strong backing. Over the next 10 years, Mr. Delors came to exert more influence over the continent’s future than any president before or since.

He fought for the European single market, launched in 1993, and the single currency, the euro, authorized by the Maastricht Treaty that year and rolled out beginning in 1999. “Those two things are his great achievements,” said Stephen Wall, a diplomat and historian who became Britain’s permanent representative to the European Union in 1995. “I don’t think they would have happened without his brain and his drive.”

The Maastricht Treaty created the European Union from the European Community that preceded it. His tenure also saw the signing of the Schengen agreement, which eliminated most border checks across much of the bloc.

He was a key combatant in conflicts that remain unresolved in the era of Brexit — Britain’s withdrawal from the European Union — on the division of power between the E.U. and national governments.

Having come to see Europe as “helpless, disabled and divided,” as he told Time magazine in 2007, Mr. Delors worked hard to win converts to his vision of integration, arguing that European countries must act together economically or plunge into decline. Another key aspect of his message — that European countries should weave a shared safety net of social polices — proved a harder sell, especially later in his tenure.

Mr. Delors was shaped by the pacifism of his father, who fought in the trenches of World War I, and by the massive dislocation of his own generation during World War II. Personally unassuming, he had the expansive political imagination of a radical. But he was a fervent advocate of prosperous stability and political centrism.

He had “an almost inhuman capacity for work,” and “a prodigious capacity to generate ideas with a pragmatic instinct for getting them implemented,” Charles Grant, former Brussels correspondent for the Economist, wrote in his 1994 biography, “Delors: Inside the House That Jacques Built.”

As European Commission president, Mr. Delors pushed single-mindedly for a Europe in which people, goods and capital could flow across borders — and for the changes that countries would need to make to prepare for the new reality. He managed to sell the idea to the French leftists who sent him to Brussels and to stalwart conservatives like British Prime Minister Margaret Thatcher, whose later clashes with him would prefigure the politics of Brexit.

Thatcher and Mr. Delors, improbable partners, worked together at first to build the single market. But Thatcher wanted to stop at what Mr. Delors saw as merely a first step toward a united continent with a single currency and liberal social policies, including bloc-wide workplace standards, job training programs and a guaranteed right to collective bargaining.

“Delors is like a sports car that went from 0 to 60 in seconds as far as she was concerned,” Wall said of Thatcher. The “nail in the coffin” of their relationship was a 1988 visit to Britain during which Mr. Delors, without first consulting her, delivered a speech in support of collective labor bargaining at the European level.

“He didn’t take soundings,” Wall said. “She overreacted.”

Her rejoinder was a resounding speech in Bruges, Belgium, in which she inveighed against a federal Europe. “We have not successfully rolled back the frontiers of the state in Britain, only to see them re-imposed at a European level with a European super-state exercising a new dominance from Brussels,” she said.

In the 1990s, even as Mr. Delors faced increasing resistance from opponents of accelerated integration, support was coalescing around his potential run for president of France. However, he decided, he said, to forgo the bid, in part because he did not want to hold office without a mandate powerful enough to institute a sweeping overhaul of society.

His legacy rested chiefly on his accomplishments at the European Union, which had 12 members in 1995 and has since more than doubled in size, with many former Eastern Bloc nations among the newer ones.

“I had not imagined we would be 27 members,” he told Time in 2007. “But I didn’t know the Berlin Wall would fall.”

Issues he argued, and battles he began, still loom over European affairs. In 2020, then-British Prime Minister Boris Johnson — who made his name at the Daily Telegraph writing alarmist articles with headlines such as “Delors plans to rule Europe” — presided over Britain’s exit from the European Union. It was the largest challenge yet to Mr. Delors’s vision.

Jacques Lucien Jean Delors was born in Paris on July 20, 1925. His father, a union activist wounded in World War I, worked as an usher and messenger at France’s central bank, where he would urge Mr. Delores to seek a career.

He did not attend the exclusive schools that traditionally funnel students into the French political elite. Seeing classmates stifled by a lack of opportunity distressed him and informed his politics for life. “I think my first sense of social injustice came when I saw my pals quitting school and going straight to the factories,” he told Newsweek.

He attended the University of Paris but soon went to work as an intern at the central bank, where he received rapid promotions. He continued to take night classes and received an economics degree.

Mr. Delors, a practicing Roman Catholic, rarely emphasized his faith, but he had religious convictions that underpinned his politics. In 1950, he became an economic adviser to a progressive Christian trade union association and went on to organize in the Christian workers movement.

In 1948, Mr. Delors married Marie Lephaille, a fellow central bank employee. She died in 2020. Their son, Jean-Paul Delors, a journalist, died of leukemia at 29, in 1982. Survivors include a daughter, Martine Aubry, the former head of France’s Socialist Party, who has been mayor of Lille, a city near France’s border with Belgium, since 2001.

Early in his career, Mr. Delors began rising through the ranks of France’s policy bureaucracy, and in 1969, he was named a top adviser to Gaullist Prime Minister Jacques Chaban-Delmas. In 1974, he joined the Socialist party. In 1979, he was elected to the European Parliament. He joined Mitterrand’s government in 1981 and, while holding a cabinet seat, was the elected mayor of Clichy, a Paris suburb, from 1983 to 1984.

At various times, Mr. Delors taught economics at the University of Paris-Dauphine and the prestigious L’Ecole Nationale d’Administration while working in government. After leaving politics in 1995, Mr. Delors ran the Jacques Delors Institute, a think tank in France.

Mr. Delors lived to see a Europe that would have been nearly impossible to imagine when he was born less than a decade after the end of World War I. But the forces of history that he marshaled to the cause of the European experiment remain in motion, their direction uncertain.

“The construction of Europe corresponds both to an ideal and to a necessity,” he wrote in 1992, in a passage quoted by Grant in the biography. “The ideal often appeared to dim [but] the necessity has always been present, even insistent for those who did not accept the historical decline of Europe.”