Under King William in 1689, Bertie was restored as a deputy lieutenant of Oxfordshire, which he held until 1701, to the commission of the peace and as a militia captain there, and as a commissioner for the assessment of both Oxfordshire and Buckinghamshire. He and Norreys were returned again for Oxford to the Convention Parliament. He was not active as a committeeman, and his time there was principally distinguished by a violent quarrel with Sir William Harbord over some remarks by the latter aimed at Bertie, about (royal) pensioners and the Westbury election. The matter ran close to a duel, and Bertie had to be secured by the serjeant-at-arms until Speaker Trevor extracted pledges from them not to renew the quarrel.