Dr Nielsen completed his primary medical degree at the University of Queensland in 1983.  He completed his Orthopaedic specialty training in Brisbane before spending 2 years overseas undergoing advanced training in hip and knee arthroplasty including revision hip and knee surgery, working both in Belfast NI and Exeter, Devon, UK.  Completing trauma fellowships in Salzburg, Austria and Louisville KY, USA, he accepted a position as a Senior Lecturer in orthopaedic surgery based at Brisbane's Princess Alexandra Hospital in 1994 until 2001. 

Dr Nielsen commenced in full time private practice in 2001 and continues that practice today in 2 locations - Mater Private and Greenslopes Private; along with maintaining an appointment at QEII hospital publically. After stepping down in his role at State Chairman of the QLD branch of the Australian Orthopaedic Association in 2018, he has completed his Graduate Certificate in Surgical Education through the University of Melbourne. 

Dr Nielsen has recently certified as an Independent Medical Examiner [CIME]. 

In November 2018 Dr Nielsen was granted the title of Associate Professor - School of Clinical Medicine by the University of Queensland and is active in developing the AOA21 program for education of Orthopaedic Trainees.