In Philadelphia he again went into business as a printer, but received little work; he printed the Lettre adressée aux habitants de la province de Québec, ci-devant le Canada (Letter to the Inhabitants of Canada) for the Continental Congress in 1775, and travelled to Montreal the following year to set up a printing press in the newly captured city.
As the Americans withdrew from Montreal, he was arrested and imprisoned, but released later in the year; however, he managed to publish several works in 1776.
In 1778 he founded the Gazette Littéraire de Montréal, edited by Valentin Jautard.
[3] Both were arrested in 1779 for sedition, and imprisoned for three years; on his release, Mesplet was $5,000 in debt but quickly dealt with his creditors.
In total, he published some seventy or eighty works, in French, English, Latin, and Iroquois; ten of these ran to more than a hundred pages, and another seven were almanacs.