It's arguably not France nor the UK, but Morocco - the Treaty of Peace and Friendship wasn't fully ratified until 1787, but Morocco was the first foreign state to recognize the United States of America (in 1777, before France) and to open its ports to "American" trade. It is the longest unbroken treaty relationship in U.S. history.

Strictly speaking, the first Treaty with Morocco was more of a non-aggression pact than a military alliance. It is true that the first active alliance was with France during the war for independence.

The relationship between both France and the UK with the US could fill books.

But suffice to say that the outbreak of the French Revolution so soon after the US, became a major political divide, especially once it turned violent in the Reign of Terror. And the Revolutionary Government and the US found themselves butting heads so much so that each nation deployed ships to the Caribbean and a few small battles erupted during the Quasi War.

Then several decades and a few governments later the French this time under Napoleon III land in Mexico and put his relative Maximilian on the throne (who would later be assassinated after an uprising), and while nothing was done at the time because the US was fighting the Civil War it was noticed.

Just as the war ends major Union forces are deployed to the Texas border and ready to invade of France doesnt scale down their forces in the US enforcing the Monroe Doctrine.

And while the US and France find themselves allies again decades later in WW1 by then France had become firmly entrenched as the #2 global power behind Britain and the US/UK relationship had grown much closer.

Umm, source on your claim, op?

I've always heard that France was our oldest ally, period. However, Morocco was the first to officially recognize the USA as a country.

I think the French recognized USA to be a country too, but I'm sure there are logistic hurdles (maybe?) that you have to go through, to officially make your country recognize a newly formed country.