After his return from India for the final time in 1772, and having purchased the Caledon estate, Alexander entered politics. He served as Member of Parliament for Londonderry City from 1775 to 1790 and was appointed High Sheriff of Tyrone in 1780 and High Sheriff of Armagh in 1781. Alexander was a staunch Tory in his politics and had spent the staggering sum of over £600,000 on acquiring estates in Ireland, including the parliamentary borough of Newtownards in County Down, and became a strenuous supporter of the Act of Union.