Thietmar was a son of the Saxon count Siegfried I the Older of Walbeck (died 990) and his wife Kunigunde (died 997), daughter of Henry I the Bald, Count of Stade (House of Udonids). His father fought with Margrave Odo against Duke Mieszko I of Poland at the 972 Battle of Cedynia. At the time of Thietmar's birth, his family sided with the Ottonian duke Henry II of Bavaria ("the Wrangler") in his uprising against his cousin Emperor Otto II. Later, a balance was achieved; Siegfried became burgrave at Möckern and his brother Count Lothair of Walbeck served as margrave of the Northern March from 983 until his death in 1003.