John was a substantial magnate but, in terms of land held in hereditary right, not one of the first rank. Probably this situation, and the example of his father, who had risen in the kings service from, humble origins to the earldom of Essex, was the spur to his long career in the royal administration. John began that career as sheriff of Yorkshire between 1234 and 1236. Then in 1237, at the request of a Parliament which conceded the king taxation, he was added to the King's council along with the earl of Surrey and William de Ferrers. In the ensuing years he gained and retained the confidence of the king. From 1237 until 1245 he seems to have acted as one of the stewards of the kings household. He was thus well fitted for his long stint as justiciar of Ireland (1245-56).
In the political crisis of 1258, however, John FitzGeoffrey was one of the kings chief opponents. Later The Westminster Flores Historiarum named him and Simon of Montfort as the ringleaders of the revolution. Certainly he was one of the seven magnates whose confederation in April 1258 began the process of reform. He was then one of the twelve chosen by the barons to reform the realm, and one of the council of fifteen imposed on the king by the Provisions of Oxford. John's sudden death on 23 November 1258 thus deprived the new regime of one of its bastions. He was a man of considerable parts, respected both by his fellow magnates and by the king. Despite his role in the revolution of 1258, when Henry III heard of John's death he ordered a solemn mass to be celebrated for his soul and donated a cloth of gold to cover his coffin. John was succeeded by his son, John FitzJohn, who became a leading supporter of Simon of Montfort.
Source www.thepeerage.com from The Dictionary of National Biography, Oxford Univ. Press
http://onlinebooks.library.upenn.edu/webbin/metabook?id=dnb