At the Council of Clermont in 1095, Adhemar showed great zeal for the crusade, and having been named apostolic legate by Pope Urban II, he accompanied Raymond IV, count of Toulouse, to the east. While Raymond and the other leaders often conflicted with each other over the leadership of the Crusade, Urban felt that Adhemar was the true leader, reflecting what Urban believed to be a spiritual campaign.
Adhemar negotiated with Alexius I Comnenus at Constantinople, re-established at Nicaea some discipline among the crusaders, and was largely responsible for the successes at Antioch. During the Crusaders' siege of that city, Adhemar organized a barefoot procession around the walls, as Joshua had done at Jericho in the Bible. After the capture of the city in June, 1098, and the subsequent siege led by Kerbogha, Adhemar organized another procession through the streets. He also had the gates locked so that the Crusaders, many of whom had begun to panic, would be unable to desert the city. He was extremely skeptical of Peter Bartholomew's discovery in Antioch of the Holy Lance, especially because he knew such a relic already existed in Constantinople; however, he was willing to let the Crusader army believe it was real if it raised their morale.
When Kerbogha was defeated, Adhemar organized a council in an attempt to settle the leadership disputes, but he died of the plague on August 1, 1098. The disputes among the higher nobles went unsolved, and the march to Jerusalem was delayed for months. However, the lower class foot soldiers continued to think of Adhemar as a leader; some of them claimed to have been visited by his ghost outside Jerusalem, and reported that Adhemar instructed them to hold another procession around the walls. This was done, and Jerusalem was taken by the Crusaders in 1099.