Since 1986 Dr FitzJohn has been a consultant radiologist for the Capital and Coast District Health Board. He was a radiologist and Managing Director of Wakefield Radiology from 1992 to 2004, before becoming Chair of Pacific Radiology, and has been Chair of Cyclotek Pharmaceuticals since 2010.
Dr FitzJohn oversaw the installation of the first MRI scanner in the greater Wellington region and introduced PACS electronic image storage to New Zealand. He initiated radiology involvement in the treatment of vascular anomalies and since 1996 has been co-Chair of the Centre for the Study and Treatment of Vascular Birthmarks. He was a central figure in introducing positron emission topography (PET) scanning to New Zealand, establishing the first PET scanner in Wellington.
In 2016 Dr FitzJohn received the award of Officer of the New Zealand Order of Merit for his services to radiology.