Prince Wilhelm was born in Rotenburg an der Fulda, the eldest child of Prince Chlodwig of Hesse and his wife, Princess Caroline of Solms-Hohensolms-Lich.
From a non reigning branch of the House of Hesse, Prince Wilhelm was closely related to the reigning Darmstadt line through his aunt, Princess Eleonore, who was married to Ernest Louis, Grand Duke of Hesse.