Brittain's parents were Protestants, but at the age of sixteen Thomas became a Catholic.
Shortly after his conversion he went to Picardy to pursue his studies, and later joined the Dominicans at Bornheim, near Antwerp, where he made his profession 22 October 1767.
In 1790 the doctor's cap, with title of Master of Sacred Theology, was conferred on him.
Father Brittain secured a foundation for the sisters at Hartpury Court near Gloucester.
On 3 May 1814, Brittain was elected provincial of the Dominicans, an office he held for four years.