Justine joined the police force on the graduate entry scheme in 1988, going into the Greater Manchester Police (GMP). After a series of promotions during her time in Manchester she moved to Merseyside Police as a Superintendent in 1999. She returned to Manchester in 2003 and in 2007 became Assistant Chief Constable.
In 2008, Justine had what was probably the most challenging year of her service. In March, the Chief Constable of GMP, Michael Todd, died suddenly under challenging circumstances and Justine worked with the rest of the command team to ensure the Force and its staff were able to get through this difficult time. In May, she was Gold Commander for the UEFA Cup Final between Zenit Saint Petersburg of Russia and Rangers FC at the City of Manchester Stadium, where a significant number of fans rioted in the city centre after the game. In June, a firearms officer was fatally shot during a training exercise and Justine led on this critical incident. With all three of these incidents occurring within a ten week period, not only her policing skills but also her resilience were severely tested.
She moved to Tayside police in 2009 and went on to become Chief Constable. During her time in this role, she achieved unprecedented performance results, most of which exceeded the Scottish average, and improved on public satisfaction rates while also reducing the budget by 8%.
Justine joined Humberside Police as Chief Constable on 1 April 2013