When the First Crusade was preached, in 1096, Baldwin sold most of his property to join, taking his wife with him. When the King of Hungary demanded a hostage to ensure the good conduct of the Crusading army, Baldwin volunteered and remained with the King until the army had left his lands.
When Tancred of Apulia broke away from the main army, to march on Cilicia, Baldwin went with him. He eventually dispossessed Tancred of the captured city of Tarsus (Tancred eventually moved on to attack Antioch), then (after the death of his wife) moved on, at the invitation of the ruler of Edessa, moved to that city, as heir, and shortly afterwards as first Count of Edessa. He married an Armenian wife, acted as intermediary between the Crusaders and the Armenians, and sent aid to the crusaders beseiging Antioch, as well as expanding his County by judicious military actions.
In 1099 he visited Jerusalem, where his brother Godefroi now ruled, before returning to Edessa, and becoming overlord of Melitene. After Godefroi's death, in 1100, he was invited back to Jerusalem. He granted Edessa to a cousin, but on his arrival learned that there was a split in influence between a secular group who wished him to succeed to the Kingdom of Jerusalem, and atheocratic group, led by the Patriarch, Dagobert, who wanted to city ruled by the Pope's representative.
Baldwin undertook an expedition against Egyptian territory, and on his return the patriarch grundinging crowned him King (although in Bethlehem, since he felt it improper to crown a king in Jerusalem itself).
Eventually Dagobert was first suspended, then deposed. Baldwin ruled as a Crusader king, undertaking multiple expeditions to expand the Kingdom. He also remarried, having abandoned his Armenian wife, thus technically committing bigamy. The technical sin preyed on him, and he was also to abandon this wife, and to go to war with the Muslims of Egypt, possibly as penance. During this campaign he died, allegedly of a surfeit of fish.