On completion of his apprenticeship, William moved to Birmingham to work for Richard Tutin, a builder and surveyor.

During this time he revised his title to architect, however, a depression in the city forced the closure of the firm and he moved to Leamington.

In 1843, during a depression in the British building industry, he emigrated to Canada with his wife and 10 children to Toronto, where his career flourished.

They were responsible for work such as the interior decorations of St. Paul's Cathedral, stone and stucco ornamentation inside St. Michael's Cathedral Basilica and stone carving, including the coat of arms in the central gable, at Bishop's Palace, and exterior embellishments on St. Lawrence Hall.

Thomas is sometimes incorrectly credited with the architectural design and the elaborate stone carvings on Victoria Hall in Cobourg, Ontario.