In 1096 he joined the First Crusade with his brother Godfrey and Eustace III of Boulogne. On the way through Asia Minor, he separated from his brothers and accompanied Tancred on a march into Cilicia, but later skirmished with Tancred's forces after taking Tarsus away from him in September 1097. Baldwin was invited to Edessa, where he was adopted by Thoros of Edessa, and then succeeded him after Thoros was assassinated in March 1098, becoming the first Count of Edessa, where he ruled until 1100.

In the meantime, the crusaders had captured first Antioch and then Jerusalem, with his brother Godfrey as ruler. When Godfrey died in July 1100, Baldwin was invited to Jerusalem by the supporters of a secular monarchy. Although initially opposed by Tancred and the new patriarch, Dagobert of Pisa, he was crowned the first king of Jerusalem at Bethlehem on 25 December 1100.

Baldwin fought many battles with the Egyptians and other Muslim neighbors and successfully expanded the kingdom including the capture of Arsuf and Sidon. He died on 2 April 1118, being succeeded by his cousin Baldwin of Bourcq.