James Watt married his cousin, Margaret (Peggy) Miller. They had five children together, but only two, James Jr. and Margaret, survived to adulthood.

Margaret Miller Watt died, leaving Watt a widower with young children. This was a difficult time for Watt, both personally and professionally.

Watt married Ann MacGregor, with whom he found companionship and support in his later life. They did not have any children together.

Watt maintained close friendships with many leading intellectuals and scientists of his time, including Joseph Black, Erasmus Darwin, and Joseph Priestley. These friendships fostered intellectual exchange and contributed to his scientific pursuits.

Watt spent his later years in retirement at his Heathfield Hall estate near Birmingham, enjoying the company of his family and friends, and continuing to pursue his scientific interests. He lived comfortably, having secured financial stability through his steam engine inventions.

James Watt died at his home, Heathfield Hall, on August 25, 1819, at the age of 83. He was buried in the churchyard of St. Mary's Church, Handsworth, near Birmingham.