His research and clinical interests are in bipolar disorder and depression, with a particular focus on: predictors of the development of bipolar disorder in at-risk individuals; molecular genetics of bipolar disorder; pharmacological and psychological treatments for bipolar disorder and depression; clinical phenomenology of bipolar disorder and depression; and neurostimulatory therapies for depression. Professor Mitchell has published over 450 peer-reviewed papers or book chapters and is lead investigator on an Australian NHMRC-funded Program Grant on depression and bipolar disorder. He serves on the editorial boards of ‘Psychiatric Genetics', ‘CNS Drugs', ‘CNS Spectrums', ‘Frontiers in Behavioral and Psychiatric Genetics', ‘Future Neurology', ‘International Journal of Bipolar Disorders', ‘Medicine Today' and ‘Therapeutic Advances in Psychopharmacology'.
In 2002 he was awarded the Senior Research Award of the Royal Australian and New Zealand College of Psychiatrists (RANZCP). In 2004, he received the Founders Medal of the Australasian Society for Psychiatry Research. In 2008, he was invited to give the endowed Samuel Novey Lecture in Psychological Medicine at Johns Hopkins University, Department of Psychiatry and Behavioral Sciences. In the 2010 Australia Day honours list Professor Mitchell was appointed as a Member of the Order of Australia for service to medical education, particularly in the field of psychiatry, as an academic, researcher and practitioner, through contributions to the understanding, treatment and prevention of mental illnesses. In 2013 he was awarded the College Citation of the RANZCP for exceptional service to psychiatry.