In the meantime his friend lord H. Petty, after having taken his degree at Cambridge, and visited the continent, returned to England, and was immediately elected one of the two representatives of Calne. In the new parliament just then convoked, this young nobleman soon began to be considered a very able and formidable ally of the opposition; and upon the final success of Mr Fox’s party, lord Henry Petty found himself, at the very early age of twenty-one, chancellor of the exchequer, a member of the privy council, and M. P. for the university of Cambridge. In this commanding situation he strongly recommended his young Scottish friend to the notice of his coadjutor, as a gentleman whose principles, character, and talents eminently fitted him for supporting the new ministry. Mr Horner was accordingly brought into parliament for the borough of St Ives in 1806. By the dismission of the Foxo-Grenville administration, Mr Horner was for a time deprived of his parliamentary seat; but the talents and integrity which he had exhibited while in office, pointed him out to the friends of liberal principles as an ally too important to be consigned to oblivion. Accordingly, on the retirement of viscount Mahon from the representation of Wendover Mr Horner was immediately nominated for that place, and soon afterwards was appointed one of the commissioners for investigating the claims on the late Nabob of Arcot, whose debts had been guaranteed by the East India Company, -- an office of considerable emolument but proportionate labour. This situation, however, he afterwards resigned, though receiving little or no emolument from professional business, which indeed he did not aim at acquiring. Once established, however, in parliament, Mr Horner continued gradually to acquire the confidence of the house, and that hold upon public opinion, without which no member of the British senate can be an efficient statesman. His speeches were little remarkable for ornament, or in a high degree for what is generally called eloquence; but he brought to the examination of every subject the power of a clear and matured understanding; and as he made it a point never to address the house upon any subject of which he had not made himself fully master, he never failed to command attention and respect. The excellence of the speaker consisted in accurate reasoning, logical arrangement of the facts, and clear and forcible illustration.

On the 1st of February, 1810, Mr Horner entered upon that part of his parliamentary career in which he reaped his most brilliant reputation. The extraordinary depreciation of the paper-currency, and the unfavourable state of the exchanges for the last two years had attracted the attention of the best economists of the day, and engaged Messrs Mushet, Ricardo, and Huskisson, and many others, in the investigation of the general principles of circulation, and of the various results which are occasioned in different countries by the variations in their respective currencies. This was a subject upon which Mr Horner felt himself at full liberty to enter. He had early turned his attention to economical subjects, and had given the result of his inquiries to the public in various articles which he contributed to the Edinburgh Review, which had attracted very considerable notice from their first appearance. Accordingly, pursuant to notice, he moved for a variety of accounts and returns, and during the spring of that year, called the attention of the house at different times to the important subject of the circulating medium and bullion trade. At the same time that Mr Horner was establishing his reputation as an economist, he neglected not the other duties of a statesman. On the 10th of May, 1810, when Alderman Combe made a motion censuring the ministers for obstructing the address of the Livery of London to his majesty in person, we find Mr Horner supporting it in the following constitutional terms: "He considered it as a question of vital importance, respecting which ministers had attempted to defend themselves by drawing the veil from the infirmities of their sovereign. It was the right of the Livery of London, as it was of other subjects, to have access to his majesty’s person in the worst times, -- even in these of Charles II. these had not been refused. The most corrupt ministers indeed, had no idea it would ever be refused. How complete would have been their triumph if they had discovered the practice which of late had prevailed! The obstruction of petitions was a subversion of the fundamental law of the land." Towards the conclusion of the same session, the house marked its sense of Mr Horner’s superior information by placing his name at the head of "the bullion committee." Mr Horner presided for some time as chairman of that committee during the examination of the evidence, and drew up the first part of the report; the second was penned by Mr Huskisson; and the third by Mr Henry Thornton. They reported "that there was an excess in the paper circulation, of which the most unequivocal symptoms were the high price of bullion, [Gold had attained a maximum of 15 1/2 per cent above the mint price.] and next to that the low state of the continental exchange; [The exchanges on Hamburg and Amsterdam had been depressed towards the latter end of 1809, from 16 to 20 per cent, below par; while the exchange on Paris was still lower.] that the cause of this excess was to be found in the suspension of cash-payments, there being no adequate provision against such an excess, except in the convertibility of paper into specie; and that the unfavourable state of the exchange originated in the same cause, and was farther increased by the anti-commercial measures of the enemy." They added "that they could see no sufficient remedy for the present, or security for the future, except the repeal of the law suspending the cash payments of the bank; this, they thought, could not be safely done at an earlier period than two years from the time of their report; but they recommended that early provision should be made by parliament for this purpose." This report excited much discussion both within and without the walls of the house. The press swarmed with pamphlets on the present state of the currency, and the remedies proposed; -- the journals teemed with dissertations on the same subject; -- the comparative merits of a metallic and a paper currency formed the topic of discussion in every company;—ministers opposed the committee’s proposition; -- and finally, Mr Vansittart, at the head of the anti-bullionists or practical men, as they called themselves, got a series of counter-resolutions passed after four nights’ keen discussion, in which the speeches of Mr Horner and several other members extended to three hours’ length.

Although defeated in their struggle, the appearance which Mr Horner made in it, was so highly respectable as to deepen the impression which his talents and knowledge had already made on the house; and from this period he appears to have exercised very considerable influence with all parties. Indeed, the urbanity of his manners, and the moderation with which he pressed his own views, were such as secured for him the respect, at least, of those from whom he differed in opinion; and while steadily and consistently supporting the party to which he belonged, he displayed a spirit of tolerance towards his opponents which totally subdued any thing like personal animosity on their part. His efforts were then often more successful than those of more gifted men, who, with greater talents, have nevertheless greater prejudice, frequently amounting to personal dislike, to struggle against. It has been supposed that had Mr Horner been in parliament after the death of Mr Ponsonby, he would have become the leader of the opposition. But for an honour so great as this, providence had not destined him. Constant application to business and the increasing weight and multiplicity of his engagements, at last overpowered a constitution which never was very strong. Indications of pulmonary consumption soon appeared, and immediate removal to a warmer climate was deemed necessary by his physicians. Crossing, therefore, to the continent, he passed through France and entered Italy; but the seeds of mortal disease had begun to spring before he took farewell of his own country, and he expired at Pisa, on the 8th of February, 1817, in the 38th year of his age. His remains were interred in the Protestant burying-ground at Leghorn, which also contains the ashes of Smollett.

On the occasion of a new writ being moved for the borough of St Mawes, which Mr Horner had represented, the character of the deceased member was elegantly sketched by lord Morpeth, and eloquent and affecting tributes of respect paid to his memory by several of the most distinguished members of the house.

A contemporary, who was acquainted with Mr Horner, both at school and at the university, thus expresses his opinion of him: "The characteristics of Mr Horner’s mind, if I apprehend them rightly, were clearness of perception, calmness of judgment, and patience of investigation: producing as their consequences, firmness of conduct and independence of principles. Carrying these qualities into public life, he evinced greater moderation and forbearance than are often found in the narrow and comparatively unambitious strifes of a less extended scene. He entered parliament at rather an early age, and soon became not only a useful and conspicuous man of business, but drew more respect to his personal character, and was regarded by both orders of the House of Commons with greater confidence and interest, than any young member had attracted, perhaps, since the early days of Mr Pitt. This will appear higher praise when it is added, with truth, that no man coming into that house under the patronage of a whig nobleman could have acted with greater liberality towards extended ideas of popular right,—with more fairness and firmness to the persons of his opponents,—or with more apparent latitude of individual judgment, on some of the most trying occasions, in all those scenes that have occurred in our recent parliamentary history. As a public speaker, he was not remarkable for the popular graces and attractions. If eloquence consists in rousing the passions by strong metaphors,—in awakening the sympathies by studied allusions,—or in arresting attention by the sallies of a mind rich in peculiar associations, Mr Horner was not eloquent. But if eloquence be the art of persuading by accurate reasoning, and a right adjustment of all the parts of a discourse, by the powers of a tact which is rather intellectually right than practically fine, Mr Horner was eloquent. He spoke with the steady calmness of one who saw his way on principle, while he felt it simply and immediately, through sobriety of judgment and good conduct; and never seemed to be more excited by his subject, or more carried away in the vehemence of debate, than to make such exertions as left one uniform impression on the minds of his hearers that he spoke from an honest internal conviction and from a real desire to be useful. In private life, he was distinguished by an impressive graveness which would have appeared heavy, had it not been observed in permanent conjunction with an easy steadiness of conversation, and a simplicity of manners very far from any thing cold, affected, or inelegant. His sense of honour was high and decided. His taste for literature, like his taste for conduct, was correct. As his acts of friendship or of duty were done without effort or finesse, so did be enjoy with quietness and relish those tender and deeply felt domestic affections which can sweeten or even adorn almost any condition of life. He was not fitted to win popularity, but his habitual moderation,—his unaffected respect for every thing respectable that was opposed to him,—and the successful pains which he took to inform himself well on the grounds and nature of every business in which he bore a part, gained him an influence more valuable to a man of judgment, than popularity."