It is known that the Norman king was the illegitimate son of the previous duke of Normandy and a woman name Herleva, whose father was a tanner. Joshua Gelernter examines some circumstantial evidence that she might have been Jewish:
According to the 19th-century historian Edward Freeman, William the Conqueror, who was also known as William the Bastard, was sometimes known as “William the Mamzer.” This is where things get interesting. Mamzer is a Hebrew pejorative meaning the child of an illegitimate sexual relationship, such as adultery or incest. In Europe, it may have entered the vernacular as a pejorative for the offspring of a particular type of illegitimate relationship—one between a Christian man and a Jewish woman. . . . [Furthermore], in medieval Europe, tanners were frequently Jews. . . .
We know that in the 11th century Normandy was home to a large number of Jews. By some estimates, its capital Rouen began that century with a population that was one-fifth Jewish. And we know that William was relatively fond of the Jews: after his Conquest, he suggested Normandy’s children of Israel resettle in England—a surprising invitation, even for someone interested in developing trade and finance. . . .
Does any of this prove that England as we know it was founded by a Jew? Of course not. But it’s something to chew on.