During or soon after his father's rebellion against King Stephen in 1143–1144, young Geoffrey was sent or made his way to Devizes, a base of the Empress Matilda.
After the earl's death, the empress recognized the right of Geoffrey III to the earldom of Essex and the vast Mandeville holdings.
The earl served as an itinerant royal justice with Richard de Lucy in 1165–1166, visiting many of the counties of England.
In 1166 he was engaged in preparations for a royal campaign in Wales when he fell ill and died in September or October.
Geoffrey was succeeded by his brother, William de Mandeville, 3rd Earl of Essex.