The OED's earliest quotation is from around 1856.
I seem to recall that a condom in French is a capote Anglaise = English overcoat, which adds further confusion.
which makes me wonder if 17th century warfare was a little different from how I imagined it. I thought they used muskets.... traces the word derivation to a Colonel Condum of Britain's Royal Guards. This authority notes that the colonel devised the 'French letter' early in the mid-17th century to protect his troops from the French.
The fraternal warmth between the French and the English is famous, of course
Euphemistic meaning "bad language" (pardon my French) is from 1895. Used in many combination-words, often dealing with food or sex. French dressing recorded by 1860; French toast is from 1630s. French letter "condom" (c.1856, perhaps on resemblance of sheepskin and parchment), French (v.) "perform oral sex on" (c.1917) and French kiss (1923) all probably stem from the Anglo-Saxon equation of Gallic culture and sexual sophistication, a sense first recorded 1749 in the phrase French novel.
To take French leave, "depart without telling the host," is 1771, from a social custom then prevalent. However, this is said to be called in France filer à l'anglaise, literally "to take English leave."