Dalkeith was returned to Parliament for Marlborough in 1793, a seat he held until 1796, and then represented Ludgershall until 1804, Mitchell between 1805 and 1806 and Marlborough again between 1806 and 1807. The latter year he was summoned to the House of Lords through a writ of acceleration in his father's junior title of Baron Scott of Tyndale. He was also Lord-Lieutenant of Selkirkshire between 1794 and 1797, of Dumfriesshire between 1797 and 1819 and of Midlothian between 1812 and 1819. He was appointed a deputy lieutenant of Northamptonshire on 9 May 1803. In 1812 he was made a Knight of the Thistle. He succeeded his father in the dukedom the same year and one of his first acts was to commission what is now the oldest iron bridge in Scotland.