When Prince Doran Martell withdrew to the Water Gardens in 298 AC, he left Manfrey as castellan of Sunspear. Although Prince Oberyn Martell had command in Doran's absence, Manfrey helped to govern with Maester Myles, Ricasso the seneschal, and the treasurer Alyse Ladybright.
A Feast for Crows
After her coup is prevented, Princess Arianne Martell is taken into the custody of Ser Manfrey at Sunspear. He informs her that her companions will be taken to Ghaston Grey and then walks Arianne to her pleasantly-appointed cell in the Spear Tower.
Manfrey is a cousin of Prince Doran Martell to an unknown degree.