Whig a candidate for Perth, in opposition to James Murray of Strowan, brother of the Duke of Atholl, but was defeated by a majority of only six votes out of 100. He remained active in the political field but withdrew shortly before the general election of 1774, having become engaged to the Honorable Mary Cathcart, second daughter of the 9th Lord Cathcart, whose sister was betrothed to the 3rd Duke of Atholl's son and heir. Two years later in 1774 he married Mary and on the same day her elder sister became Duchess of Atholl. "Jane," wrote Lord Cathcart, "has married, to please herself, John, Duke of Atholl, a peer of the realm; Mary has married Thomas Graham of Balgowan, the man of her heart, and a peer among princes." A solicitous husband, when his wife discovered on the morning of an Edinburgh ball that she had left her jewel-box at Balgowan, Graham rode the to and from Balgowan using relays of horses to ensure that she would have her jewellery at the ball. Her portrait by Thomas Gainsborough was highly acclaimed when exhibited at the Royal Academy in 1777. The painting Portrait of Mrs Mary Graham was covered over by Graham after her death. The painting now hangs in the National Galleries of Scotland, Edinburgh. Graham spent the next eighteen years as a quiet country gentleman, spending his time on riding and sports, studying the classics and making occasional visits to London and Edinburgh. When his carriage was stopped in Park Lane, London by a highwayman demanding money, jewels, and watches at gunpoint, while two accomplices seized the horses' heads, Graham, who was on the opposite side of the carriage, leapt across the ladies to the carriage-door, and collaring the assailant, threw him to the ground. Then, drawing his sword, which at the time formed part of a dress suit, he threatened to run the man through if his associates, who were holding the horses' heads, attempted to come to his assistance. They immediately fled, and the prostrate highwayman was arrested. Death of his wife Mary Graham's health began to decline, and in the spring of 1792, on the recommendation of her medical adviser, she went to the south of France with her husband and sister. However, during the voyage she died off the coast near Hyères, France, on 26June 1792. Her sorrowing husband hired a barge to take