Gillies was an interior designer/decorator who apprenticed with the architect and interior designer Frederick Deane's practice in Sydney known as Deane and Hall (Don). In the 1950s, Gillies and Deane combined their practice in Edgecliff, a Sydney suburb as "Thomas Gillies". Gillies continued the practice under his name as various locations. In the 1970s, he moved to Burleigh Heads, Qld, then back to Sydney.