He first rose to prominence in the Kingdom in 1170, when he married Agnes of Courtenay, who had been married three times before: firstly to Reginald of Marash, who left her a widow; secondly (possibly bigamously) to Amalric, count of Jaffa and Ascalon and future king of Jerusalem, with whom she had two children, Baldwin IV and Sibylla; and thirdly to Hugh of Ibelin, her fiancé or husband before her marriage to Amalric.

Some writers have claimed that the marriage between Agnes and Reginald of Sidon was annulled as well, as they were related within the prohibited degrees, but this is based on a misinterpretation of William of Tyre, who says Gerard discovered the relationship between "the two aforementioned people".

A bailli, or regent, needed to be appointed, and Reginald was among the supporters of Raymond III of Tripoli (Amalric's first cousin) over Miles of Plancy for this post.

To boost the hope of military support from his cousin Henry II of England and to reduce Raymond of Tripoli's influence, in 1180 Baldwin had married his widowed sister Sibylla to a Poitevin noble, Guy of Lusignan, a vassal of the Angevins, whose older brother Amalric had already established himself at court.

As the king's health failed, he appointed Guy regent in 1183 during Saladin's invasion, although Raymond and his allies were extremely hostile towards him.

The thirteenth century Old French Continuation of William of Tyre, also known as the Chronicle of Ernoul, claims that he was in the process of negotiating its surrender to Saladin when Conrad of Montferrat arrived.

He offered to retire to Damascus and convert to Islam, but it was all a ruse: he was only wasting time in order to allow the defenses of the castle to be strengthened.

He supported the annulment of the marriage of Humphrey IV of Toron and Isabella of Jerusalem, so that Isabella could be married off to Conrad of Montferrat; the anonymous author of the Itinerarium Peregrinorum et Gesta Regis Ricardi calls Reginald a member of a "council of consummate iniquity" (together with Balian of Ibelin and Maria Comnena and Payen of Haifa) for supporting this act.

This did not endear him to those western chroniclers who supported Richard and Guy of Lusignan: like Raymond of Tripoli, he was even (falsely) accused of having secretly converted to Islam.

After his release from captivity in 1190, he married Helvis of Ibelin, daughter of his friends Balian and Maria, and over 40 years his junior (she was probably born in 1178).