Dallas’s father, descended from a cadet of Dallas of Cantray and son of an excise officer at Elgin, first appeared in the London trade directories about 1757 ‘behind the Royal Exchange’, together with George Dallas. He became an insurance broker in partnership with Allen and afterwards with Hicks, in Mincing Lane. His eldest son was ‘intended from his infancy for the bar’ and initiated himself as a public speaker at Coachmakers’ Hall, where he was ‘allowed by his auditors to be a very correct and eloquent speaker’.
Dallas acquired much parliamentary business as counsel to committees on contested elections, but claimed that he had refused the offer of a seat. In 1802, however, he came in, as a friend of Addington’s administration, for a Cornish seat at the disposal of government. Addington considered him (or perhaps his brother George) for Indian office in September 1802, but he did not obtain it. In November 1803 he was described as ‘solicitor elect in case of vacancy’. Fox described him in March 1804 as Addington’s only unpaid defender, who yearned to be solicitor-general. In his maiden speech (by which he got ‘great credit’), 24 May 1803, he had defended Addington’s foreign policy; on 2 Feb. 1804 he spoke against Fox’s motion on the Middlesex election petition.
On Pitt’s return to power, Dallas joined with Addington in opposing his additional force bill in June 1804, preferring the latter’s defence scheme. On 22 June he defended the lord advocate against his critics. On 28 Dec. he addressed a long letter to Addington pointing out that he had been returned as a friend of government and that the change of administration had reduced his chances of professional promotion; this placed him in a predicament and he saw no point in continuing in Parliament, unless Addington came to terms with Pitt. When Addington did so in January 1805, Dallas obtained the office of chief justice of Chester. According to Robert Ward, this was Pitt’s own idea.
In July 1806, when it was proposed to appoint two salaried assessors to the courts of prize appeals and plantation causes, Dallas was mentioned as being ‘eminently qualified, because so large a portion of his practice has been in those two courts, and his general manner is so respectable and decorous. He would also have the chief justiceship of Chester to resign, if that should be required, and he would be willing to accept the assessorship on those terms.’ Apart from acting as counsel for the slave owners at the bar of the House, 20 Feb. 1807, he remained at Chester and became celebrated there because he would not allow felons to suffer capital punishment if he could prevent it, ‘though for crimes for which other judges would urge their execution’.
In 1813 after his ‘long absence’ at Chester, Dallas was recommended by the lord chancellor to be solicitor-general in Liverpool’s administration. Writing to Lord Sidmouth, who he knew would approve, he confessed that he had ‘very considerable struggles with myself before I could resign an office which I acquired thro’ your lordship’s kind partiality, and which, to me, was a security for the future and a source of very considerable present satisfaction’. Unprovided with a seat in Parliament, Dallas was transferred a few months later to common pleas, where he was ‘much liked on account of his gentlemanly manners, though not considered so profound a lawyer [as Abbott]’. After taking part in the Luddite trials in October 1817, Dallas became chief justice in the following year, with the Prince Regent’s goodwill.