Media cameras flashed and onlookers cheered as Jane Hartley dropped her ballot into a wooden box taking pride of place on the bar.
A gaggle of Americans from New Jersey and New York followed with their votes as the narrow mahogany-panelled watering hole began filling up with its usual mix of regulars and tourists.
The "straw poll" dates to 1924, decades before Americans could send in absentee ballots, to allow expatriates to have at least a symbolic say in the outcome of their national election.
Bartender Gilles Chauvin, a 32-year veteran at Harry's, sported a Hillary Clinton sticker on his white jacket as well as an oversized talking Donald Trump pen in his lapel pocket.
Honored to cast 1st vote in Harry's New York Bar's Straw Poll. 5% of Americans abroad who could vote did so in 2012. Let's do better. Vote. pic.twitter.com/KXWPWXsa5d— Jane Hartley (@USAmbFrance) October 5, 2016
Campaign posters for Clinton and Trump as well as a variety of also-rans hung from the once cream-coloured ceiling turned brown from tobacco smoke.
The saloon, reconstructed piece by piece after being shipped from New York, was a favourite of expats such as Ernest Hemingway and F. Scott Fitzgerald.
The owner, Isabelle MacElhone, the widow of Harry's grandson Duncan, said Hartley was the first special guest in the bar's history to launch the straw poll.
US citizens must show their passports to be allowed to take part, with the ballots to be kept until election day itself on November 8th, when the results will be announced.
The legendary birthplace of the Bloody Mary prides itself on the fact that it has failed only twice to predict the correct outcome, in 1976 when Democrat Jimmy Carter won and in 2004, when Republican George W. Bush was re-elected.
For her part, the ambassador told AFP: "My prediction is that the American public will make the right choice."