Charlie has a background in criminology, undertaking research at the University of Cambridge and then the Home Office, where (from 1987) he undertook research on crime and managed a programme of research on drugs prevention.

He then moved to the Joseph Rowntree Foundation in 1999, where he managed research programmes on young people, drugs and alcohol. In 2010, he moved to the Department of Health Sciences at the University of York, undertaking research and teaching on drug and alcohol use. He joined the Social Policy and Social Work Department at York in 2017 (now the School for Business and Society), focusing on teaching and research on criminal justice responses to drug use.

Over his career, Charlie has undertaken research on rape offences, reconviction rates, prison, probation and, most notably, drug use. Here he has focused on the risk factors for addiction, the stigmatisation of problem drug users, through-care of prisoners with drug problems and police responses to drug users.