Francis Hopkinson was one of America's founders -- a "Founding father." He was not a "key founder" however (meaning that he's one that most people don't remember).
He may also have been a unitarian. What I have discovered over the many years of researching America's founders personal religious views is that virtually all of were in some way formally connected with a church that had a Trinitarian creed. But we really don't know what they believed in until we dig a little deeper into the record.
Theological unitarianism was "en vogue" amongst some of the more "philosophically minded" theists.
I came upon a passage from Hopkinson where I thought he identified himself as an anti-Trinitarian. However, reading the passage in context Hopkinson appears to be speaking of someone else, perhaps a fictitious character. A "cobbler."
As he wrote:
ON my arrival in this country, I was much chagrined to find that the doctrine of the Trinity was generally received and professed; a doctrine against which I had acquired the strongest antipathy from my master, the great philosopher. However, as war raged in the country, there was no time for theological disquisitions; and as the justice of the war was clearly on the side of the Americans, I resolved to take an active part in their cause, and enlisted myself a soldier in the Pennsylvania line. I went through inexpressible toil and danger, in marches, counter-marches, skirmishes and battles, and was finally wounded at the siege of York-Town, when lord Cornwallis and his army surrendered to the allied forces of France and America.
After the peace, I supported myself by cobling and patching, and employed my leisure hours in combating the doctrine of the Trinity, and devising grand schemes for public utility.
See also here.