Dr Riordan undertook training in rheumatology in Australia and Oxford. He was awarded the Michael Mason award to study crystal arthropathies in Bristol, UK. During his more than twenty five years as a consultant rheumatologist in Wollongong he has developed expertise in most areas of clinical rheumatology, especially inflammatory joint disease and connective tissue diseases. He has retained a specific interest in crystal arthropathies whilst developing extensive experience and interest in giant cell arteritis (temporal arteritis) and large vessel vasculitis in addition to bone disease and osteoporosis. He undertakes teaching of general medicine and rheumatology to medical students, interns, physician trainees, and advanced trainees in rheumatology (for the Royal Australian College of Physicians). He is also head of the Department of Rheumatology at Wollongong Hospital.