After joining the 1st Foot Guards at the age of 15 as an ensign he went on to serve in Flanders in 1794, by which time he had achieved his promotion to lieutenant. In 1798, he took part in the unsuccessful landing at Ostend. In the Peninsular War, he served at both the Battle of Vigo, and at Corunna, for which he was awarded a medal. He took part in the Walcheren in 1809. During the later stages of the Peninsula War was second in command of his regiment at Cadiz, and later at the Battle of Seville.
He served with distinction at Quatre Bras and the Battle of Waterloo. Promoted in early June (3 June 1815) to major general, he was assigned to the First Corps, under overall command of the Prince of Orange. On 18 June, the day of Waterloo, he commanded two battalions of the 1st Foot Guards, each 1000-men strong and led the Guards in repelling the final assault of the French Imperial Guard. For his service at Waterloo, Maitland was created a Knight Commander of the Order of the Bath, (KCB) on 22 June 1815, the Dutch Order of Wilhelm and the Russian Order of St. Vladimir. For their part, the 1st Foot Guards were granted the honorary title of 'First or Grenadier Regiment of Foot Guards'.
He was appointed lieutenant governor of Upper Canada in 1818 and supported the Family Compact that dominated the province. He attempted to suppress and reform pro-American tendencies in the colony and resisted demands of radicals in the government. In his role Maitland was the first to propose the civilizing techniques that would eventually lead to the establishment of the Canadian Indian residential school system. He believed that while a shift from hunting to agricultural pursuits would assist with civilizing Indigenous populations, it was gaining the influence of children that would lead to success. In an 1820 report to the Colonial Office he argued for the introduction of industrial schools to minimize the children's exposure to the savage influence of their families.:12–17:55–57 His tenure in Upper Canada ended in 1828 when he was appointed lieutenant-governor of Nova Scotia serving there from 1828 until 1834.
Maitland went to India and became commander in chief of the Madras Army in 1836 serving for two years. In 1843 he was appointed Colonel of the and in 1844 Governor of the Cape Colony, but was removed during the Xhosa War. He is still highly respected in the Kingdom of Lesotho for his judgment on the border issue between the Orange River Afrikaners and the Basotho of King Moshoeshoe I, which, had it been implemented, would have secured the economic future of the kingdom. He was made a Knight Grand Cross of the Order of the Bath on 6 April 1852.