Thomas Fiott de Havilland (1775-1866), military engineer and architect, was born at Havilland on 10 April 1775, the eldest son of Sir Peter de Havilland and his first wife, Carterette. He entered the Military Corps of Madras Engineers as a cadet in 1793. He was promoted to the rank of ensign on 3 May 1793, and took part in the siege of Pondicherry later that year. During the Ceylon expedition of 1796, he was given the temporary rank of adjutant, though still an ensign. He stayed on the island until receiving orders to return to Madras, where he arrived on 29 May.
On returning to Madras, De Havilland was promoted to lieutenant on 1 June 1796. In 1799, as commanding engineer, he accompanied Lt. Colonel Brown's force of four thousand men, which marched on Trichinopoly in support of the campaign against Sultan Tippoo. In 1801 he went with the Indian expeditionary force to Egypt as field and, later, commanding engineer. His services in Egypt ended in July 1802, when it is presumed he returned to Guernsey. In January 1804, during his voyage back to India, he was captured by a French privateer, though he was released on parole, and proceeded to Madras, where he resumed his rank later in April. In April 1805 he was appointed engineer at Hyderabad, and during 1805-1806 was engaged in carrying out a survey of the northern boundaries of the Nizam's territories. This brought him recognition with the higher authorities in Madras, and on 6 January 1806 he was promoted to captain. In 1808 he married Elizabeth de Sausmarez. Whilst stationed with the garrison at Seringapatam, he was one of the officers dismissed from the service in 1810 following the dispute between the Governor of Madras, Sir George Barlow, and the local East India Company officers. He was subsequently reinstated in 1812.
In tandem with his military career, De Havilland was also a distinguished civil engineer and architect. In 1814 he was appointed superintending engineer and architect to the Madras Presidency. In this capacity, he built a number of important buildings in the city, including St George's Cathedral, St Andrew's Presbyterian Church, the Mount Road, in which he owned several properties, and, in 1822, the Madras Bulwark or sea wall. He was promoted to the rank of major in the Madras Engineers on 8 October 1815, and on 9 February 1821 he was appointed acting chief engineer of the Corps. His official report to the governor on the future composition of the Corps, 23 November 1821, led to several important changes. His acting post as chief engineer ended on 10 January 1823.
De Havilland returned to Guernsey in January 1824. He was promoted to lieutenant colonel on 1 May, but retired from the service on 20 April 1825. He spent the remainder of his life in his native island. In 1829 he rebuilt Havilland Hall, and in 1842 he was elected a Jurat of the island's Royal Council. He died at De Beauvoir, Guernsey, on 23 February 1866.