[N3062] Herleva's background and the circumstances of William's birth are shrouded in mystery. The written evidence dates from a generation or two later, and is not entirely consistent, but of all the Norman chroniclers only the Tours chronicler asserts that William's parents were subsequently joined in marriage. According to Edward Augustus Freeman, the Tours chronicler's version cannot be true, because if Herleva married the Duke, then William's birth would have been legitimized, and thus he would not have been known as William the Bastard by his contemporaries.

The most commonly accepted version says that she was the daughter of a tanner named Fulbert from the town of Falaise, in Normandy. The meaning of filia pelletarii burgensis is somewhat uncertain, and Fulbert may instead have been a furrier, embalmer, apothecary, or a person who laid out corpses for burial. Statue of Arlette in Huy, Belgium where the mother of William the Conqueror is considered a child of the country.

Some argue that Herleva's father was not a tanner but rather a member of the burgher class. The idea is supported by the appearance of her brothers in a later document as attestors for an under-age William. Also, the Count of Flanders later accepted Herleva as a proper guardian for his own daughter. Both of these would have been nearly impossible if Herleva's father had been a tanner, which would place his standing little above that of a peasant.

Orderic Vitalis described Herleva's father Fulbert as the Duke's Chamberlain (cubicularii ducis).

According to one legend, it all started when Robert, the young Duke of Normandy, saw Herleva from the roof of his castle tower. The walkway on the roof still looks down on the dyeing trenches cut into stone in the courtyard below, which can be seen to this day from the tower ramparts above. The traditional way of dyeing leather or garments was to trample barefoot on the garments which were awash in the liquid dye in these trenches. Herleva, legend goes, seeing the Duke on his ramparts above, raised her skirts perhaps a bit more than necessary in order to attract the Duke's eye. The latter was immediately smitten and ordered her brought in (as was customary for any woman that caught the Duke's eye) through the back door. Herleva refused, saying she would only enter the Duke's castle on horseback through the front gate, and not as an ordinary commoner. The Duke, filled with lust, could only agree. In a few days, Herleva, dressed in the finest her father could provide, and sitting on a white horse, rode proudly through the front gate, her head held high. This gave Herleva a semi-official status as the Duke's concubine. She later gave birth to his son, William, in 1027 or 1028.

Some historians suggest Herleva was first the mistress of Gilbert of Brionne with whom she had a son, Richard. It was Gilbert who first saw Herleva and elevated her position and then Robert took her for his mistress.