On 24 December 1264 he was summoned to a Parliament convened by Simon de Montfort, which was held in 1616 to give a precedence of 1264 to the Lordship de Ros of Helmsley. He sided with Simon de Montfort in the Barons' War. On 14 August 1265 he was pardoned for his opposition to King Henry III. He was Commissioner in the North of England to ensure Aid was yielded to the King in 1268.
He was married to Isabel d'Aubigny, rich heiress and granddaughter of William d'Aubigny. They lived at of Belvoir in Leicestershire and reportedly had eight children:
1. William de Ros, 2nd Baron de Ros (1255–1317).
2. Isabel de Ros (c. 1244 - June 12, 1356. Married de Fauconberge.
3. Mary de Ros (1245 - May 23, 1326). Married William de Braose, son of John de Braose and Margaret ap Llewellyn. Margaret was a daughter of Llywelyn the Great.
4. Joan de Ros (c. 1252 - October 13, 1348). Married John Lovell, 1st Baron Lovell of Tichmarch.
5. Avelina de Ros. Married Sir John de Bohun of Midhurst. Alleged eighth-generation ancestor of Daniel Boone.
6. Robert de Ros (1265–1361). Married "Ernberge".
7. John de Ros, Bishop of Carlisle (d. 1332).
8. Nicholas de Ros.
Sir Robert de Ros 
