By the 13th century Thornbury was sufficiently important to become a Borough and it retained that status up to 1883 until when the town was administered by a Mayor and 12 Aldermen. Some of the Borough property of those days, including an oak coffer dated 1615 containing the municipal scales, weights and measures, passed into the custody of the Thornbury Town Trust and is now held by the Town Council. The Local Government Act of 1894 made provision for the election of Parish Councils and, at a meeting held at the Cossham Hall on 4 December that year, no less than 39 candidates were nominated, some several times. Four withdrew before voting, which was by a show of hands, but after this lengthy process a poll was demanded and an election subsequently held on 17 December 1897 for the 11 Council seats.

On local government reorganisation in 1974, the then Parish Council was able to regain Town status and use is now made of civic regalia at appropriate times. The Town Mace is in silver gilt and was made in 1889. It is carried by the longest-serving Councillor, who precedes the Mayor on ceremonial occasions. The mace is a replica of the original solid silver one, dated 1698, which is in the Bristol Museum. The Mayoral Chain, created in silver by local craftsman, Mr John Pinch, was a gift to the town from the Armstrong Trust and was presented by the late F D Armstrong, Esq, in April 1975. The chain worn by the Mayor's escort was presented by the Thornbury and District Chamber of Trade, and the Deputy Mayor's chain was presented by Mr Maurice Taylor, Baron Portlethen of Thornbury Castle in 1990.

The administrative offices of the Town Council are situated at the Town Hall and are open Monday to Friday from 9 am to 5 pm. The building, which was constructed in 1860 was originally a police station with a court and, later a magistrates court only. The building was later acquired by the Town Council and renovated to a high standard to form office accommodation, council chamber and community facilities.