He became deeply entwined in the financial lives of his clients, who put him in charge of their charities, placed his name on deeds for their properties and let him control their savings.

Mr. Epstein profited from associations with retail magnate Leslie Wexner, financier Leon Black, Johnson & Johnson heiress Elizabeth Johnson and hedge-fund billionaire Glenn Dubin, among others. He pitched clients tax-saving strategies, handled prenuptial agreements, estate planning and other personal matters, and inserted himself into one of the largest hedge-fund deals on record, increasing his wealth each step along the way.

Mr. Epstein earned at least $200 million from Mr. Wexner alone, according to estimates by people familiar with the relationship. For more than 15 years, a power-of-attorney document allowed Mr. Epstein to act on Mr. Wexner’s behalf in legal or financial dealings.

A detailed account of his professional life, based on legal documents and interviews with a wide range of his associates, shows how Mr. Epstein, a college dropout from Coney Island in Brooklyn, also benefited from good timing and a knack for self-promotion, latching onto deep-pocketed clients as their wealth soared and markets surged.