Barnett entered the Lombard Street, London banking partnership of Barnett, Hoare and Hill in 1790. He joined the Whig Club, 7 Nov. 1797, and in 1804 was a steward for the election of Sir Francis Burdett for Middlesex. In 1806 he came in for Rochester under the aegis of a fellow Whig John Calcraft, after a ruthless contest in which he gave as good as he got.
Barnett again contested Rochester when Sir Thomas Boulden Thompson was obliged to seek re-election in 1816 and narrowly defeated him, but his election was voided. He was not opposed at the fresh election. He was one of the steadiest voters with opposition in the next two Parliaments. He supported Catholic relief, 9 May 1817. On 21 Mar. 1817 he spoke against the bill for Exchequer bills, which would add ‘still further to the unfounded debt of the country, amounting now as it did to nearly 50 millions’. On 18 Feb. 1818 he spoke on the Election Law amendment bill: he regularly supported parliamentary reform. He wound up the opposition to the ‘iniquitous’ indemnity bill, 11 Mar. 1818.
At the election of 1818 there was a rift between him and Calcraft, Barnett insisting that he had never indicated any intention of standing down in Calcraft’s favour at the dissolution.
