Dr Riordan has provided care to patient with rheumatic disorders in the Illawarra for over twentieth years. He has expanded his interest in crystal arthritis whilst also promoting the management of osteoporosis in the area. More recently, he has developed a particular interest in giant cell arteritis and vasculitis. His practice centres predominately on inflammatory joint disease and unusual rheumatic disorders.

Dr Riordan is also actively involved in the University of Wollongong teaching medical students. He also teaches junior doctors. He is head of the Department of Rheumatology at Wollongong Hospital and is responsible for training the advanced trainee in rheumatology. He is a senior member of The Friday Group involved in weekly robust peer review.

Dr Riordan is also the convenor of The Australian Rheumatology Review Group which publishes reviews of important topics in rheumatology.