Nixon attended Macquarie University before attaining a Master of Public Administration from Harvard University's John F. Kennedy School of Government.
The daughter of Ross Nixon, an assistant commissioner with the New South Wales Police Force, Christine Nixon began her policing career with the same police force in 1972, also rising to the rank of Assistant Commissioner.
She was appointed Chief Commissioner of Victoria Police in April 2001 by the Bracks Labor government.
Having initially set a retirement date of late March 2009, Nixon departed earlier at the request of the Victorian Government to take on responsibility for the Victorian Bushfire Reconstruction and Recovery Authority.
She was succeeded as Chief Commissioner by Simon Overland.
As Chief Commissioner, Nixon marched in uniform during Melbourne's gay and lesbian 'Pride March', run as part of the Midsumma Festival. Nixon is heterosexual but marched to express her support for gay and lesbian causes, stating "What I'm doing is supporting decent and reasonable people who want to get on with their lives, and they have been treated appallingly previously by the Police, and I'm prepared to do something about it. And if it's a small symbol of marching with them, then that would be a reasonable thing to do."
Nixon was criticised when she joined her husband, John Becquet, a former Qantas senior executive of crew operations, on the inaugural international flight of Qantas' Airbus A380 airliner from Melbourne to Los Angeles, as guests of Qantas in an all-expenses-paid, three-day trip. Ms Nixon called the trip "reasonable" and commented that she was accompanying her husband, and that she had not had a holiday in about 12 months. Mr Becquet defended the trip saying they were invited to LA after a chance meeting with a Qantas executive. "I am normally her handbag but on this she's my handbag." The couple celebrated their 16th wedding anniversary during this week. The head of the Office of Police Integrity, Michael Strong, sought information on the matter from Ms Nixon at a time of calls for her to reimburse the airline for the cost of the trip.
Nixon was called to appear before the 2009 Victorian Bushfires Royal Commission concerning her actions as chief commissioner during the Black Saturday bushfires on 7 February 2009. Counsel assisting the commission, Rachel Doyle SC, questioned Nixon on issues such as a morning hairdressing appointment, a lunchtime meeting with her biographer and an evening dinner at a restaurant, all during the worst day of the bushfires. Nixon defended her actions, stating "It was not my job to swoop in and take control. When you have good people who are more skilled in emergency management than I am, you let those people do the job."
In February 2009 Nixon assumed responsibility for the Victorian Bushfire Reconstruction and Recovery Authority, the agency tasked with rebuilding areas affected by the Black Saturday bushfires of February 2009. She stood down from the position on 17 July 2010, and announced she would take on a voluntary advisory role with the authority.
Nixon was a non-executive director of Foster's Group from 1 April 2010 to 31 August 2010. She also serves as Patron or Advisor to the Alannah and Madeline Foundation, Onside Victoria, Operation Newstart Victoria and The Phoenix Club Inc. She has been appointed as the Deputy Chancellor of Monash University.
Nixon has been awarded the Australian Police Medal, the National Medal, the Centenary Medal, the New South Wales Police Medal and clasps for Ethical and Diligent Service and the New South Wales Police Force Olympic Citation.
As a morbidly obese woman, Nixon's size has received disproportionate emphasis in the media.
On 18 July 2010 Nixon was taken to hospital. She subsequently underwent gall bladder surgery.
In 2011 Nixon published her memoirs in the book Fair Cop (ISBN 9780522856859), launched by the Prime Minister, Julia Gillard. The book was criticised by the Police Association Victoria for what the association claims is a biased recall of events, and by the Herald Sun.