Baldwin was ransomed by his cousin, Joscelin of Courtenay, Lord of Turbessel, in the summer of 1108. Tancred attempted to retain Edessa, but Bernard of Valence, the Latin Patriarch of Antioch, persuaded him to restore the county to Baldwin. Baldwin allied with Jawali, but Tancred and his ally, Radwan of Aleppo, defeated them at Turbessel. Baldwin and Tancred were reconciled at an assembly of the crusader leaders near Tripoli in April 1109. Mawdud, the Atabeg of Mosul, and his successor, Aqsunqur al-Bursuqi, launched a series of campaigns against Edessa in the early 1110s, devastating the eastern regions of the country. Baldwin accused Joscelin of treason for seizing the prosperous town of Turbessel from him in 1113 and captured the neighboring Armenian lordships in 1116 and 1117.
Baldwin of Boulogne, the first king of Jerusalem, died on 2April 1118. He bequeathed Jerusalem to his brother, Eustace III of Boulogne, stipulating that the throne was to be offered to Baldwin if Eustace failed to come to the Holy Land. Arnulf of Chocques, the Latin Patriarch of Jerusalem, and Joscelin of Courtenay, who held the largest fief in the Kingdom of Jerusalem, convinced their peers to elect Baldwin king. Baldwin took possession of most towns in the kingdom and gave Edessa to Joscelin. After the army of the Principality of Antioch was almost annihilated on 28 June 1119, Baldwin was elected regent for the absent Bohemond II of Antioch. The frequent Seljuq invasions of Antioch forced him to spend most of his time in the principality, which caused discontent in Jerusalem. After Nur al-Daulak Balak captured him in April 1123, a group of noblemen offered the throne to Charles I, Count of Flanders, but Charles refused. During his absence, the Jerusalemite troops captured Tyre with the assistance of a Venetian fleet. After he was released in August 1124, he tried to capture Aleppo, but al-Bursuqi forced him to abandon the siege in early 1125.
Bohemond II came to Syria in October 1126. Baldwin gave his second daughter, Alice, in marriage to him and also renounced the regency. Baldwin planned to conquer Damascus, but he needed external support to achieve his goal. He married off his eldest daughter, Melisende, to the wealthy Fulk V, Count of Anjou in 1129. The new troops who accompanied Fulk to Jerusalem enabled Baldwin to invade Damascene territory, but he could seize only Banias with the support of the Nizari (or Assassins) in late 1129. After Bohemond II was killed in a battle in early 1130, Baldwin forced Alice to leave Antioch and assumed the regency for her daughter, Constance. He fell seriously ill in Antioch and took monastic vows before he died in the Holy Sepulchre. Baldwin had been respected for his military talent, but he was notorious for his "love for money".
Baldwin was a younger son of Hugh I, Count of Rethel and Melisende of Monthléry. He was closely related to the lords of Courtenay and Le Puiset, and other noble families in the Ile-de-France. He was also a kinsman of the brothers Eustace III of Boulogne, Godfrey of Bouillon, and Baldwin of Boulogne, but their exact relationship is unknown. Historian Alan V. Murray says that the primary sources suggest that their connection "was not particularly close", and that Baldwin was most probably related to their mother, Ida of Lorraine.Thomas Asbridge says that Baldwin was the Boulogne brothers' second cousin.
He was the lord of Bourcq when he joined the army of Godfrey of Boulogne at the beginning of the First Crusade. The army departed for the Holy Land on 15August 1096, and reached Constantinople on 23 December. The Byzantine emperor, Alexios I Komnenos, urged the crusader leaders to take an oath of fealty to him. Godfrey of Bouillon appointed Baldwin, Conon of Montaigu and Geoffrey of Esch to represent him at a meeting with Alexios in January 1097. After Godfrey and his principal commanders swore fealty to the Emperor, the crusader army was shipped to Asia Minor in February.
Baldwin of Boulogne and the Norman Tancred broke away from the main army to invade Cilicia around 15September 1097. Baldwin accompanied them in Boulogne's contingent. He also participated in Boulogne's military campaigns against the Seljuq rulers of the fortresses on the plains near the River Euphrates. After seizing Ravendel, Turbessel and Edessa, Boulogne established the first crusader state, the County of Edessa, in early 1098.
Baldwin rejoined the main crusader army, which was marching towards Jerusalem, near Tyre in late May 1099. He and Tancred seized Bethlehem; there was no resistance as the town was inhabited by local Christians. The crusaders laid siege to Jerusalem, and shortly afterwards Baldwin and Tancred captured an elderly Muslim nobleman. After he refused to convert to Christianity, Baldwin's soldiers beheaded him at the Tower of David to frighten the defenders of Jerusalem. Jerusalem fell to the crusaders on 15July. Baldwin left Jerusalem in the retinue of Robert II, Count of Flanders, in late August. Robert returned to Europe, but Baldwin remained in Syria. Geoffrey of Bouillon died on 18July 1100. Baldwin of Boulogne decided to return to Jerusalem to take possession of Geoffrey's inheritance.