Called to the bar, Lockhart practised in the Marshalsea and Palace courts and at the Oxford sessions. He became a distinguished member of the Oxford circuit, remarkable for his ‘extraordinary patience in investigation, and great retentiveness of memory’. The same circuit provided him with an heiress for his wife and a seat in Parliament. He was a member of the Constitutional Society and on 23 Apr. 1790 chaired their meeting at the Crown and Anchor in favour of parliamentary reform. In 1796 he acted as election agent at Oxford for the wealthy London merchant Henry Peters; on 25 July 1801, during the latter’s absence and illness, he offered himself at Oxford independently, claiming to have had ‘for many years the friendship of the freemen’.
Lockhart gave a general support to government until the session of 1810. From the start he was an unabashed contributor to debate, with a bias towards legal questions. Sheridan clearly thought him pert (13 Aug. 1807) in his confidence that his reputation had preceded him to the House. He first spoke in favour of the militia transfer bill, 27 July 1807. On 6 August he obtained leave to suspend the Act of 35 Geo. III on ecclesiastical benefices. It was when, a week later, he held forth on the state of Ireland that Sheridan thought he exceeded his brief. He defended the Copenhagen expedition, 8 Feb. 1808. He was hostile to Catholic claims, 11 May. He defended corporal punishment in the army, 30 June 1808, and the militia enlistment bill, 2 Feb. 1809. He questioned the leading figures in the case against the Duke of York in February 1809 and on 17 Mar. pronounced the duke innocent of corruption, but guilty of immorality. He believed this warranted public rebuke, 20 Mar. He retained his youthful advocacy of parliamentary reform, describing Curwen’s bill as inadequate, 18 May: ‘he had always been of opinion that a moderate reform was necessary’. He counted out the House in the debate on the cruelty to animals bill, 13 June. In the debate on public economy, 19 June, he claimed that merchants wished to see more, not fewer, officials employed in the Customs.
After voting with ministers for the address on 23 Jan., Lockhart deserted them on the Scheldt question, 26 Jan., 23 Feb., 30 Mar. 1810. He was opposed to Burdett’s committal to the Tower, 5 Apr., and spoke in favour of adjournment next day (with the opposition). On 13 Apr. he was in the minority for Irish tithe reform. He voted against sinecures, 17 May, and for parliamentary reform, 21 May; also for Tierney’s motion on the application of the droits of Admiralty. He objected to the Berkshire petition for Burdett’s release: ‘its object was libel’, 6 June. He gave his ‘steady support’ to the election bribery bill, 20 June. The Whigs were not surprisingly ‘hopeful’ of him in 1810, but he rallied to ministers on the Regency, 1 Jan. 1811. On 21 Mar. he supported the liberty of the press—but not in India. He opposed Romilly’s dwelling house robbery bill, 9 Apr. He was critical of the misconduct of Richard Mansel Philipps and Benjamin Walsh and on 31 Jan. 1812 attempted to secure the exclusion of bankrupts from the House. He did not himself succeed in this, any more than he did with his proposal of June 1810 to amend the Poor Laws by doubling the qualification for parish settlement, or in his bid in the session of 1811 to secure statutory registration of charities to avoid abuses of them: but with Romilly’s assistance he carried the latter (29 Apr. 1812), with a separate bill providing summary remedies against abuses (52 Geo. III, c.101).
Lockhart survived a severe contest in 1812 by forming a coalition with John Atkyns Wright against a Whig candidate. He was listed a Treasury supporter and until 1815 usually was one. He opposed Catholic relief throughout in 1813 (and thereafter) as he had promised to do when the Oxford petition against it was presented, 1 Dec. 1812. He voted for Christian missions to India, 1 July 1813. His speeches were for the first two sessions confined largely to legal matters or tinkering with legislation. He strongly objected to the abolition of attainder for treason, though not for felony, 5 Apr. 1813, 25 Apr. 1814. He was a spokesman for the American loyalists’ compensation claims, 20 May 1813. He was anxious to help devise a new insolvent debtors bill but dissatisfied with the result (1813-16).
It was at this point that Lockhart seems to have decided that ministers were not paying sufficient attention to economy. He opposed the new Post Office, 1 June, and an increase in judges’ salaries, 6 June 1815. He was critical of foreign alliances, 12 June, particularly of the raising of subsidies for Russia by Dutch loans. In the debate on the address, 2 Feb. 1816, and again on 28 Feb. he called for fiscal relief for the landed interest. On 29 Feb. he presented a London petition against the property tax, which his constituents had directed him to oppose [1 May 1815]. He agreed to support it, if modified, 6 Mar. He opposed the army estimates, 28 Feb., 6 and 11 Mar. He seems to have been wavering politically. He was reported as saying at this time that although Castlereagh was ‘a very able and fully qualified man for his situation as ministerial leader in the House of Commons’, he thought highly of Henry Brougham and of George Tierney.
Lockhart voted against ministers on the composition of the finance committee, 7 Feb. 1817, for the reduction of the Admiralty establishment, 17, 25 Feb., and for a committee on the Bank, 19 Feb. But he disliked the tone of petitions for parliamentary reform, 6, 11 Feb., and supported the suspension of habeas corpus, 22 Feb., 24 June 1817, 29 Jan., 13 Mar. 1818. He opposed the repeal of usury laws, on behalf of the landed interest. Objecting to the expense of the Millbank penitentiary, 4 Mar. 1818, he suggested that a vigorous police force to combat crime would be better: ‘instead of criminals being punished, society was punished; and society was punished because it was so negligent’. He was in the majority against the Duke of Clarence’s marriage grant, 15 Apr. True to the principles of his youth, he voted for the repeal of the Septennial Act, 19 May. He was opposed to imprisonment for the publication and sale of radical literature, 21 May. He was teller for Brougham’s motion for inquiry into popular education, 3 June. The same day he reported as chairman of the committee to investigate the brewers’ monopoly in London, against which he had presented a petition; the brewers were exonerated.
On the hustings in 1818 Lockhart admitted that his line on the Corn Laws, the property tax and the suspension of habeas corpus had offended his supporters, but was not deterred by his defeat then from offering again in 1820. Nothing came of a report that he was to come in elsewhere in 1818. He was admired for
the affability of his manners, his companionable talents, delightful temper and unconquerable good humour: his ripe and well-cultivated experience in regard to the duties of the House of Commons; the promptitude, steadiness and alacrity of his habits in matters of business; and above all, his sound and sincere attachment to the cordial principles of our constitution in church and state.
The Late Elections (1818), 253; Kenyon mss, Kenyon to Stockdale, 11 July [1818].
He retired from politics after a further defeat in 1830 and died recorder of Oxford, 13 Aug. 1835.