Mordaunt spent much time in the gallery of the House while his father was county Member and reported debates to his family, his Household place detaining him in London. His interest in politics met with a rebuff in 1774 when he was defeated in a bid to succeed his father as county Member. He was returned unopposed in 1793, but by then had no ambition to cut a figure at Westminster. He was regarded as spokesman for the interests of Birmingham, which had rejected him in 1774. He esteemed Pitt and gave a general support to administration. On 9 May 1793 he supported the bill to enable Hemlingford Hundred (which included Birmingham) to raise a loan towards the damages due to victims of the Birmingham riots of 1791. Letters to his wife show that he was an anxious man and a conscientious attender; he reported to her the debates of 21 and 31 Jan. and 10 Mar. 1794. At that time, his colleague being injured, he was left to attend to all the Warwickshire business. On 25 Nov. and 3 Dec. 1795 he disparaged the petition from Birmingham against the treason bills presented by Sheridan: if it had not been coupled with a prayer for peace, it would have received scant support. On 18 Feb. and 15 Mar. 1796 he voted for the abolition of the slave trade.
Mordaunt attended the opening of Parliament, but obtained five weeks’ leave of absence on 8 Nov. 1796 for the recovery of his health. He took the ministerial side on the county petition of June 1797.