The founding date of Wennigsen is unknown. It possibly dates back to 5th to 8th century. The first dated occurrence (1199 to 1206) is a copy of a certificate from bishop Hartbert of Hildesheim. At this time there was a settlement of Bernhard, earl of Poppenburg, in Wennigsen. He had three roses each with five leaves in his coat of arms. Today Wennigsen bears one of them in its coat of arms.
Duke George William of Hanover (later of Brunswick-Lüneburg) split the Großvogtei Calenberg and founded a new Amt Calenberg in which Wennigsen became court place. The Magistrates' Court of Wennigsen still exists.