Due to his activity as a diplomat he knew personally the most important people both on the Arab and the Christian sides. His autobiography, Kitab al-I'tibar, gives a good idea of the lifestyle of the time and of the relationship between the Christians and Muslims.
In 1138 he travelled under the order of the Mameluk Moinuddin Unur, the governor of Damascus, to Jerusalem, to discuss with King Fulk the possibility of an alliance against Zengi, the emir of Mosul. Usamah was received hospitably and he was able to establish a pact of mutual assistance between Unur and the Franks against Zengi. In accordance with the pact, the fortress of Banias was transferred to the Franks. Usamah also later accompanied Unur on a state visit to Jerusalem.
The short period of Frankish-Damascene cooperation gave Usamah the opportunity to become more familiar with the Franks. With a remarkably even-handed approach, Usama comments on their practices. Raised in a highly regulated and sophisticated Islamic society, the Christian practise of trial by ordeal appeared particularly strange to him, as did the christians non-restrictive treatment of women. Much has been made of his apparent distaste for frankish medicine, but this stems from a highly selective and rather unscholary (preferring a second-hand account to a firsthand account) reading of the text - Usama's second-hand description of "strange" frankish medicine is something he clearly distinguishes from his own firsthand experiences with frankish treatments, which he in facts enthusiastically recommends and describes - although he certainly finds them odd. Also, it has to be remembered that Usama's autobiography is written within the adab (courtly) literary genre, which stresses using contrasting viewpoints and does not necessarily always demand that the viewpoints be factual. The bravery and careful discipline of the Christian warriors impressed him (he once remarked that this was the only virtue of the Franks). However, he politely declined the offer of a Frankish knight to take his son to Europe in order to educate him about the virtues of chivalry. As a whole, though, he is not overly hostile toward the Franks, naming several of them friends.
He later entered the service of Zengi, and then the Egyptian Fatimids. The Fatimids sent him as an ambassador to Nur ad-Din to negotiate an alliance against the Franks, although these negotiations failed. On his return journey he remained for two years in Ascalon, which was under siege by the Franks. Usamah helped organize Ascalon's resistance to the siege, although the city eventually fell.
After earthquakes in Syria in 1156 and 1157, Usamah, who was at that time in Damascus, lost nearly his entire family. The cities of Aleppo, Tripoli, Beirut, and Homs were heavily damaged, but the worst destruction was in Hama and Shaizar. Usamah's cousin, the emir Muhammad ibn Sultan, was celebrating the circumcision of his son with his family and the nobles of Shaizar as the walls of the city collapsed on them. Only the princess of Shaizar was saved from the rubble. This earthquake marked the end of the independence of the emirate, which soon fell into the hands of the Hashshashin and in 1157 was conquered by the Franks.