At the Council of Clermont in 1095, Adhemar showed great zeal for the crusade (there is evidence Urban II had conferred with Adhemar before the council). Adhemar was named apostolic legate and appointed to lead the crusade by Pope Urban II on 27 November 1095. In part, Adhemar was selected to lead because he had already undertaken a pilgrimage to Jerusalem in 1086 and 1087. Departing 15 August 1096, he accompanied Raymond IV, Count of Toulouse, to the east. Whilst Raymond and the other leaders often quarrelled with each other over the leadership of the crusade, Adhemar was always recognized as the spiritual leader of the crusade.
Adhemar negotiated with Alexius I Comnenus at Constantinople, reestablished at Nicaea some discipline among the crusaders, fought a crucial role at the Battle of Dorylaeum and was largely responsible for sustaining morale during the siege of Antioch through various religious rites including fasting and special observances of holy days. After the capture of the city in June 1098, and the subsequent siege led by Kerbogha, Adhemar organized a procession through the streets, and 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 on 1 August 1098, probably of typhus. 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 during the siege of 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.