| Constituency | Dates |
|---|---|
| Middlesex | 1784 – 1802 |
First prothonotary of the court of common pleas 1768 – 94; chairman Mdx. and Westminster quarter sessions 1781 – 1816; bencher, L. Inn 1795, treasurer 1804.
Belittled in political circles as ‘the son of a bricklayer’,1Francis Newdigate to Sir Roger Newdigate, 24 Mar. 1784, Newdigate mss. and by George III as that of ‘some tradesman’,2Diaries of Sylvester Douglas, i. 394. Mainwaring was in fact the son of a wealthy architect and surveyor. He became prominent in Middlesex, defeating Sir John Hawkins, Dr. Johnson’s friend, in the election for chairman of the quarter sessions. Mainwaring, like his predecessors in the office, received from secret service funds an annuity of £350 (later raised to £750), virtually as a salary.
For his election for Middlesex in 1784 he received a payment of £1,000 from secret service funds.3Royal archives, Windsor. He stood jointly with John Wilkes, and headed the poll. He attended the House regularly, spoke frequently, and steadily supported Pitt. He spoke on such matters as the shop tax (which he opposed), the Westminster police, transportation of convicts, the relief of insolvent debtors, the bill for making the destroying of trees a felony, etc.; but on the great political questions—parliamentary reform, India, the Regency bill—he was silent.
He died 28 Feb. 1821.