Thomas Kennedy was born in 1726. He was the seventeenth child of Sir John Kennedy of Culzean, and the second surviving son. He inherited the baronetcy in 1744 on the death of his elder brother. He joined the British army and served in the Netherlands and in France.
He left the army in 1747 and went on a grand tour, spending time in France and Italy. Throughout this period he corresponded with the factor in charge of the Culzean estate about improvements to the Castle and its surroundings. He returned to Culzean in 1755 and set about improving the estate.
Kennedy's distant cousin, John Kennedy, eighth earl of Cassillis, died in 1758, and the title, though disputed, eventually passed to Thomas Kennedy in 1762. He took possession of Cassillis House and continued to improve both estates. Throughout his life he was engaged in risky enterprises, including stock speculation and a long career in smuggling. He also borrowed heavily to finance improvements to the estate, and died in debt in 1775.