In 1028 Baldwin married Adèle of France in Amiens, daughter of King Robert II of France; at her instigation he rebelled against his father but in 1030 peace was sworn and the old count continued to rule until his death.

During a long war (1046–1056) as an ally of Godfrey the Bearded, Duke of Lorraine, against the Holy Roman Emperor Henry III, he initially lost Valenciennes to Herman, Count of Mons. However, when the latter died in 1049 Baldwin married his son Baldwin VI to Herman's widow Richilde and arranged that the children of her first marriage were disinherited, thus de facto uniting the County of Hainaut with Flanders. Upon the death of Henry III this marriage was acknowledged by treaty by Agnes de Poitou, mother and regent of Henry IV. Baldwin V played host to a grateful dowager queen Emma of England, during her enforced exile, at Bruges. He supplied armed security guards, entertainment, comprising a band of minstrels. Bruges was a bustling commercial centre, and Emma fittingly grateful to the citizens. She dispensed generously to the poor, making contact with the monastery of Saint Bertin at St Omer, and received her son, King Harthacnut of England at Bruges in 1039.

From 1060 to 1067 Baldwin was the co-Regent with Anne of Kiev for his nephew-by-marriage Philip I of France, indicating the importance he had acquired in international politics. As Count of Maine, Baldwin supported the King of France in most affairs. But he was also father-in-law to William of Normandy, who had married his daughter Matilda. Flanders played a pivotal role in Edward the Confessor's foreign policy. As the King of England was struggling to find an heir: historians have argued that he may have sent Harold Godwinsson to negotiate the return of Edward the Atheling from Hungary, and passed through Flanders, on his way to Germany. Baldwin's half-sister had married Earl Godwin's third son, Tostig. The half-Viking Godwinsons had spent their exile in Dublin, at a time William of Normandy was fiercely defending his duchy. It is unlikely however that Baldwin intervened to prevent the duke's invasion plans of England, after the Count had lost the conquered province of Ponthieu.

Married in 1028, Amiens, Somme, Picardie, Frankrike, to Adéle av MESSINES 1009-1079 with
M Baudouin VI av FLANDERN 1030-1070
F Matilda av FLANDERN 1031-1083
M Robert I av FLANDERN 1035-1093

Gift med
Adéle av Messines, född 1009 i Melun, Seine et Marne, Ile-de-France, Frankrike, död 1079-01-08 i Messines, Västflandern, Belgien. Hertiginna av Normandie och Grevinna av Flandern.

Barn:
Robert I av Flandern, född 1035, död 1093-10-13