In August, 1783, Erskine was appointed Lord Advocate in the Coalition Ministry, in succession to Henry Dundas (afterwards Lord Melville). He held office only for a very short period in consequence of a sudden change of Ministry in December, 1783. Anticipating this, Dundas offered, on the day of his appointment, to lend him his own silk gown, suggesting it was hardly worthwhile buying a new one; Erskine replied that no doubt Dundas’s gown was made to fit any party, but that, however short his term of office might be, he declined to put on the abandoned habits of his predecessor. He was succeeded by Ilay Campbell (afterwards Lord President of the Court of Session). On 24th December, 1785, Dundas having resigned the post of Dean of the Faculty of Advocates, Erskine was elected in his place by a decided majority, in spite of the influence of the Government, which was exerted against him. Lord Cockburn remarks, “His political opinions were those of the Whigs; but a conspicuous and inflexible adherence to their creed was combined with so much gentleness that it scarcely impaired his popularity. Even the old judges, in spite of their abhorrence of his party, smiled upon him; and the eyes of such juries as we then had, in the management of which he was agreeably despotic, brightened as he entered” (“Life of Lord Jeffrey,” 1852, vol. i., p. 93). Erskine had been annually re-elected Dean of Faculty since 1785; but in consequence of his having presided at a public meeting, held in Edinburgh on 28th November, 1795, to petition against the war, his political adversaries determined to oppose his re-election; and at the meeting of the Faculty on 12th January, 1796, Robert Dundas of Arniston, then Lord Advocate, was chosen Dean. Lord Cockburn, commenting on this incident, observes--“This dismissal was perfectly natural at a time when all intemperance was natural. But it was the Faculty of Advocates alone that suffered. Erskine had long honoured his brethren by his character and reputation, and certainly he lost nothing by being removed from the official chair. It is to the honour of the society, however, that out of 161 who voted, there were 38 who stood true to justice, even in the midst of such a scene” (“Life of Jeffrey,” vol. i., p. 94). On the death of Lord Eskgrove in October, 1804, Erskine was offered the office of Lord Clerk Register, but declined it, refusing to separate his fortunes from those of his party. On the return of the Whigs to power in 1806 he once more became Lord Advocate, and was at the same time returned member for the Dumfries District of Burghs. The downfall of the Ministry in March, 1807, however, again deprived him of office, and the dissolution in the following month put an end to his Parliamentary career.