Dr Armstrong gained undergraduate medical training at Cambridge University and Guy's Hospital, London, and qualified in 1974. His rheumatological training was undertaken at Guy's Hospital with one year spent doing laboratory research under Professor G Panayi, supported by an Arthritis Research Campaign research fellowship. Dr Armstrong was appointed as consultant rheumatologist in Southampton in 1985, where he is currently lead consultant in rheumatology. Dr Armstrong also provides a service to Hythe, Lymington and Romsey and visiting rheumatology service in Guernsey on a monthly basis. Although his main interests are in inflammatory rheumatic disease, Dr Armstrong's clinical practice embraces all aspects of rheumatology including spinal pain, soft tissue rheumatism, bone disease and medicolegal practice.

Dr Armstrong also pursues an active interest in the application of computer technology to healthcare education and until 2011 worked for NHS Evidence, developing a source of high quality information to support healthcare decision-making. In the past he has authored and programmed a CD ROM on rheumatology published by Cambridge University Press and has created and maintained an educational website on rheumatology for the Arthritis Research UK (JointZone).