Baldwin was born c.1000 and was Count of Flanders from 1036 until his death.

He was the son of Baldwin IV of Flanders, who died in 1035. He, in turn, was a descendant of Ælfthryth, daughter of Alfred the Great, Saxon King of England.

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 from 1046 until 1056, as an ally of Godfrey the Bearded, Duke of Lorraine, against the Holy Roman Emperor Henry III, he initially lost Valenciennes to Hermann of Hainaut. However, when the latter died in 1049 Baldwin married his son Baldwin VI to Herman's widow Richildis and arranged that the sons 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.

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.

Baldwin and Adèle had five children:

Baldwin VI who lived from 1030 until1070

Matilda, who married William the Conqueror

Robert I of Flanders who lived from c.1033 until1093

Henry of Flanders born c.1035

Sir Richard of Flanders who lived from c.1037 until1086