Sinnamon graduated from the Architecture degree course at The University of Queensland in 1958. While studying the final years of his degree he also worked part-time with firms including Prangley & Crofts. After graduating, he won an Italian Government scholarship for postgraduate study at the University of Rome. After working briefly in Rome and London, he returned to Brisbane in 1960 and entered a short-lived partnership with John Hitch.

Sinnamon was also an advocate of heritage conservation within the built environment. He served on the National Trust of Queensland’s Listings Committee from 1972–84 and established the Trust’s Conservation and Restoration Committee in 1979. From 1988 he served on the Heritage Advisory Committee of the Ahern and Goss Governments; he was also a member of the Green Paper Committee for Heritage Legislation which led to the establishment of the Queensland Heritage Council in 1992. He also served on the Council’s Heritage Register Assessment Committee. Having been a lecturer and Head of School at the Department of Architecture at The University of Queensland, Sinnamon later became the Architecture Department’s in-house historian, compiling its history and discussing the challenges of architectural education for its fiftieth anniversary in 1987.