Between 1964 and 2000 he conducted clinical research at the Institute of Psychiatry,[1] University of London, and the Bethlem-Maudsley Hospital.
From 2000-2003 he ran a computer-aided self-help clinic at Imperial College, London, where he was a visiting professor.
Marks' research included the treatment of anxiety, phobic, obsessive-compulsive and sexual disorders; interactions between drugs and behavioral psychotherapy; development of a nurse behavioral psychotherapist training program (in relation to which he coined the term 'barefoot therapist', modelled on Mao Zedong's term Barefoot Doctor); community care of serious mental illness; health care and cost-effectiveness evaluation; and electroshock conversion therapy.
[3] He has developed computer aids both to evaluate treatment outcome and for self-help - matters which continue to be a central interest.
He was also instrumental in the creation of the self-help organisation Triumph Over Phobia and was a founding member of the BABCP.