PARIS—At the height of the financial crisis, Rothschild & Cie. assigned one of its veteran bankers to groom a new hire named Emmanuel Macron.

Mr. Macron had no experience in banking. Instead, he had powerful mentors who had recommended him to Rothschild as a danseur mondain—literally, high-society dancer—who could drum up business.

“He was identified as being a very singular person with lots of contacts,” recalls Cyrille Harfouche, the veteran assigned to shepherd Mr. Macron. By the time Mr. Macron left Rothschild four years later, he had negotiated a multibillion-dollar deal and become one of its youngest-ever partners.

Mr. Macron’s banking career followed a playbook that now has upended the political order and placed the French presidency within his grasp, with a final-round election against Marine Le Pen on May 7. Mr. Macron made friends in high places who propelled him to ever-higher echelons of French society. Along the way he acquired a repertoire of skills, from piano and philosophy to acting and finance, that helped impress future mentors.

The approach allowed Mr. Macron to shortcut the traditional political path. Rather than run for office in his hometown, gradually building a constituency, he proceeded straight to Paris, where he became an expert on banking and European technocracy. He acquired a mastery of arcane regulations, from the 3,334-page French national labor code to the plumbing of the European Union’s single market, that made him a valuable potential aide to politicians being whipsawed by the EU’s complexity and the gyrations of global markets.