He applied eyeliner as an affectation, supposedly to remind bartenders to look customers in the eye. He moved from New York City to Cornwall-on-Hudson, about 65 miles to the north. He called himself “gaz,” a singularly lowercase version of the English diminutive for Gary.

Image The Negroni — gin, Campari, sweet vermouth and an orange peel twist — was Mr. Regan’s signature drink. His book on the subject was published in 2013.

And to speed up service at a bistro in Cognac, France, he put an imprint on his signature cocktail, the Negroni (gin, Campari, sweet vermouth, an orange peel twist), by wielding one of his index fingers as a swizzle stick in a row of the drinks ready for waiting customers. (The retailer Cocktail Kingdom now sells a bar spoon in the life-size shape of his finger, either gold-plated or in stainless steel.)

In “A Proper Drink: The Untold Story of How a Band of Bartenders Saved the Civilized Drinking World” (2016), Robert Simonson characterized Mr. Regan as “a bon vivant eccentric, a devil-may-care libertine as blissfully unconcerned with his reputation as your average aging rock star.”

“He is an unlikely authority figure — more barfly than bar scholar,” Mr. Simonson wrote. “But an authority figure he nonetheless is, one of the accepted grand old men of the cocktail resurgence. He achieved this status primarily because he got there first.”

Gary Lee Regan was born on Sept. 18, 1951, in Rochdale, a town in Greater Manchester, England, to parents who ran two pubs in nearby towns, the Prince Rupert in Bolton and the Bay Horse in Cleveleys.

After training as a chef at Courtfield Catering College in Blackpool, he ran a bistro with his wife, Norma. They divorced after about two years of marriage, and he moved to New York in 1973, when he was 22.

A friend from Bolton hired Mr. Regan to tend an Upper East Side bar in Manhattan, after which he became manager of the North Star Pub at South Street Seaport. There he was schooled in the fine points of barroom banter by observing a customer, a Scotsman, who would pore over reference books on whiskey over lunch so that he could later impress friends with his knowledge.