Tweedmouth was returned to Parliament for Berwickshire in 1880, a seat he held until 1894. The seat had been held earlier in the century by his great-uncle, Sir John Marjoribanks, 1st Baronet and cousin, Charles Albany Marjoribanks. He served under William Ewart Gladstone as Comptroller of the Household in between February and July 1886 and was sworn of the Privy Council the same year. When the Liberals returned to power under Gladstone in 1892, he was made Parliamentary Secretary to the Treasury. He succeeded his father in the barony in March 1894, only a few days before Gladstone resigned and Lord Rosebery became Prime Minister. Rosebery appointed Tweedmouth Lord Privy Seal, with a seat in the cabinet, and in May 1894 he also became Chancellor of the Duchy of Lancaster. He retained these posts until the government fell in 1895. After ten years in opposition, the Liberals again came to power in December 1905 under Sir Henry Campbell-Bannerman, who appointed Tweedmouth First Lord of the Admiralty, with a seat in the cabinet. In early 1908 he was criticised for corresponding with German emperor William II on the British naval programme. The matter was referred to the House of Commons. Chancellor of the Exchequer H. H. Asquith eventually stated that the correspondence was "a purely personal and private communication, conceived in an entirely friendly spirit" and no action was taken. However, when Asquith succeeded Campbell-Bannerman as Prime Minister in April 1908 Tweedmouth was removed as head of the Admiralty and became Lord President of the Council. He suffered a nervous breakdown in June 1908, a condition which was said to partly explain his indiscretion in communicating with the German Emperor on naval matters. Although his health later recovered, he resigned in October 1908. He was made a Knight of the Thistle in 1908. An advocate of worker's rights and social legislation, Tweedmouth was supportive of the Liberal Party's alliance with the Labour Party in the lead-up to the 1906 General Election, believing that the Liberals could not win without it, and regarded as “humbug” the view that such an alliance meant class legislation. He died on 15 September 1909.
Family
Lord Tweedmouth married Lady Fanny Octavia Louise, daughter of John Spencer-Churchill, 7th Duke of Marlborough and aunt of Sir Winston Churchill, in 1873. She reportedly died from cancer in August 1904, aged 51 "at Lord Tweedmouth's Glen Affric shooting lodge". They had a son and heir; Dudley, 3rd Baron Tweedmouth. Lord Tweedmouth's parliamentary career saw him reported as being the Laird of Guisachan and Glenaffric who was, on occasions, "in a fighting mood". Following Lady Tweedmouth's death, Lord Tweedmouth sold the Lairdship of Glen Affric, the property including the Guisachan Estate and deer park that his family had owned since the 1850s. He was reported as being a "generous laird", who, like his father, "did much for the people" of his estate; the "ties which united the people of Glen Affric with the Laird and his lady were close". Lord Tweedmouth survived his wife by five years and died in September 1909, aged 60. He was succeeded in the barony by his son, Dudley.
