Present at the Battle of Stirling Bridge on 11 September 1297, he survived the defeat of the English army at the hands of the Scots under William Wallace. He was appointed, by John de Warenne, 6th Earl of Surrey the leader of the English army, to the constabulary of Stirling Castle, with Marmaduke Thweng as his deputy, following the rout of the English army. FitzWarin, with Marmaduke Thweng surrendered Stirling Castle and were committed as prisoners to Dumbarton Castle. FitzWarin was later exchanged for Henry St Clair, Baron of Roslin in a prisoner exchange.Armstrong & McBride, p. 56.
