Dr Brennan is a highly esteemed NHS Consultant in Clinical Genetics, specialising in inherited cancer, cardiovascular disease, connective tissue disorders, and endocrine genetics. He has been a pivotal figure in the Northern Genetics Service based in Newcastle upon Tyne since his appointment in 2002.

Dr Brennan's fascination with genetics began during his medical studies in the 1980s, a time when clinical genetics was an emerging field. Following a research degree focused on genes in the developing heart, he completed his medical training and pursued postgraduate training in both adult medicine and paediatrics before specialising in Clinical Genetics.

In 2008, Dr Brennan was appointed Clinical Director of the Northern Genetics Service, where he has been instrumental in managing a clinical and laboratory team dedicated to providing genetic diagnostics for the population of North East England and North Cumbria. He also played a key role in establishing a new satellite clinical genetics department in Middlesbrough.

Since 2003, Dr Brennan has served as the Clinical Lead for Cardiac and Endocrine Genetics for the Northern Genetics Service. He also held the position of Clinical Lead for Cancer Genetics from 2002 to 2020. His work is particularly focused on leveraging new technology and innovative clinical pathways to enhance diagnosis and management for individuals with rare inherited diseases.

Between 2014 and 2018, Dr Brennan was the Clinical Programme Director for the North East & North Cumbria NHS Genomic Medicine Centre. In this role, he oversaw the integration of whole genome sequencing into routine care as part of the 100,000 Genomes Project. He remains actively involved in several collaborative research projects aimed at improving diagnosis and treatment.

Dr Brennan is also a dedicated educator, known for his engaging teaching style. He lectures to a diverse range of undergraduate and postgraduate audiences both in the UK and internationally. Each year, he spends a week teaching genetics at Newcastle University's Medical School in Malaysia.

Dr Brennan's contributions to the field of clinical genetics and his commitment to education and research make him a respected and influential figure in the medical community.