His clinical interests are cancer genetics, inherited cancer, cancer screening and the introduction of tumour genetic tests into clinical cancer care. Dr Mackay established regional cancer genetics services in East Anglia and North East Thames and developed a remote cancer genetics service, using real-time teleconferencing technology, based in Great Ormond Street Hospital NHS Trust.

He was the clinical principal investigator of an evaluation of regular mammography in women under 50 with a family history of breast cancer funded by the NHS Research and Development Programme. He was a principal investigator in the UK Familial Ovarian Cancer Screening Study (UKFOCSS). He initiated the Genetic Breast Cancer Trial, which was a trial of chemotherapy in women with breast cancer, who carry a faulty breast cancer gene. This was the first randomised trial of chemotherapy based on inherited genetic make-up in the world.

He has been on the advisory board of several UK cancer charities and has acted as medical advisor to several genetic testing companies