He was the second eldest son of Hernry Borwin III and his wife Sophia (d. before 24 June 1241), the daughter of King Erik Knutsson of Sweden.
His elder brother John died before his father, so after his father's death, Waldemar inherited Rostock and ruled alone.
According to some sources, he went blind some time before his death and divided his territory between his sons.
Waldemar was married with Agnes, a daughter of Count John I of Kiel.
They had three sons: This article about a member of the German nobility is a stub.