Judith Gillespie began her policing career when she joined the Royal Ulster Constabulary (RUC) in 1982 as a police constable. During her initial training at Enniskillen, she was awarded the Baton of Honour. She was promoted to Chief Inspector in 1997, Superintendent in 1999, and Chief Superintendent in 2002.
In 2003, she attended the Strategic Command Course at the Police Staff College, Bramshill. In 2004, she was appointed Assistant Chief Constable. This made her the first female to reach chief officer rank in the Police Service of Northern Ireland. On 1 June 2009, she was promoted to Deputy Chief Constable, becoming the second most senior police officer in Northern Ireland. She served as Acting Chief Constable from August 2009 to September 2009. She retired from the police on 31 March 2014.
A keen sportswoman, she played hockey and squash for the RUC. In 2013, when the World Police and Fire Games was held in Belfast, Northern Ireland, she served as the Chair of the Board.
Mrs Gillespie was appointed an OBE in the 2009 Birthday Honours for public service. In 2014, she was promoted to CBE for services to policing and the community in Northern Ireland. In July 2012, she was awarded an Honorary Doctorate by Queen's University Belfast.
Since leaving the force after 32 years service, Mrs Gillespie has been busy serving as a member of the Equality Commission and Probation Board, alongside charity work for Marie Curie and a role as a visiting professor at The University of Ulster.
In 2016, she returned to the realm of justice and law enforcement as a board member on the Republic's newly established Policing Authority. This independent watchdog, only up and running since January, oversees An Garda Siochana.