Pembroke's career follows pretty much the same trajectory as Salisbury's. In the beginning of the play, Pembroke is a loyal subject of King John. In the second half of the play (beginning in Act IV), however, Pembroke turns against his king. The first trigger is his and his fellow nobles' anger at King John's imprisonment of Arthur. They think this reflects badly on his kingship, and, hence, on his subjects.
Once Pembroke and his fellow nobles find out that Arthur is dead, they swear allegiance to his corpse. In more practical terms, they join forces with Louis the Dauphin of France, who is invading England. Once Pembroke and his friends learn that Louis plans to have them executed, however, they abandon him and join forces with King John again. Pembroke is at King John's side when the monarch dies at Swinstead.