Blair was born in Scarborough, Ontario. Blair's father had served as a police officer for 39 years. Blair considered pursuing a degree in law or finance, when he initially enrolled at the University of Toronto Scarborough in the mid 1970s. Blair initially studied economics at the University of Toronto. He left to follow his ambition of being a police officer, but returned later and completed a Bachelor of Arts in economics and criminology.
Blair joined the Toronto Police Service while in university to make money and began taking courses on a part-time basis. Blair walked a beat near Regent Park and later worked as an undercover officer in Toronto's drug squad.
After Blair earned his bachelor's degree in criminology, he advanced his career in the police service in the late 1980s, taking part in drug busts involving the seizure of millions of dollars of cocaine. Chief David Boothby assigned Blair to improve the poor community relations between the officers of 51 Division, which patrolled Blair's old beat near Regent Park. Blair normalized police relations with the community by measures such as sending cops to read to kids in local elementary schools and engaging with local businesses and churches.
In 1999, Blair was considered as a candidate to replace outgoing chief Boothby, but mayor Mel Lastman, with the support of Premier Mike Harris, chose to hire Julian Fantino, then head of the York Regional Police. After reorganization of the senior ranks after Fantino's ascension as police chief, Fantino became head of detective operations.
Blair was selected in a 4–2 vote of the Toronto Police Services Board in early April 2005, and formally appointed Chief of the Toronto Police Service on April 26, 2005. He succeeded Mike Boyd, who had served as interim chief after the expiry of Julian Fantino's contract. Prior to his appointment as chief, Blair worked for approximately 30 years as a Toronto police officer, with assignments involving drug enforcement, organized crime and major criminal investigations. Blair served as president of the Canadian Association of Chiefs of Police. In 2007, he was appointed an Officer of the Order of Merit of the Police Forces:840–841 and in 2012, he was elevated within the Order to the level of Commander. He is a Member of the Venerable Order of Saint John.:870
At a June 28, 2010 rally, protesters called for Blair's resignation, because of the detention of nearly 1000 people during the 2010 G-20 Toronto summit protests. In a December 8, 2010 interview, Blair indicated that he would not resign, despite growing criticism of his leadership during and after the summit.
Responding to questions about a controversial regulation enacted by the Cabinet of Ontario to increase police powers during the summit, Blair was supportive, stating that "[i]t was passed in exactly the procedure as described in our legislation in Ontario". Post-summit reports revealed that on June 25, prior to the start of the summit and shortly after Blair defended a widely reported misinterpretation of the regulation in a press conference, the police department received a government bulletin clarifying the misinterpretation and explaining that the new regulation accorded them no additional power to demand identification outside of the summit perimeter. Blair's spokesperson stated that as of the press conference, Blair was unaware of the clarification; however, Blair did not retract his prior remarks to the press after receiving the bulletin. When interviewed after the summit, Blair confirmed that there was never an extraordinary legal requirement for the public to present identification within 5 m of the perimeter fence, but that he "was trying to keep the criminals out". In December 2010, following a critical report by the Ontario Ombudsman, André Marin, Blair admitted regret that he had initially interpreted the regulation at face value, and did not promptly clear up confusion about the meaning of the regulation.
In 2013, Blair came into conflict with Toronto mayor Rob Ford after confirming to the media that the police had obtained a video of the mayor smoking what appeared to be crack cocaine. Blair said he was "disappointed" in the mayor, As the investigation into the mayor and his friend Alessandro Lisi continued, Mayor Ford dared Blair to arrest him and accused him of wasting money in their surveillance of Ford. Councillor Doug Ford claimed Blair had “gone rogue” and violated the Police Services Act when speaking out about the mayor during the ongoing police investigation. On August 11 2014, Blair served councillor Doug Ford with notice of defamation. Doug Ford accused the police chief of using the suit as "payback" in retaliation against the mayor for not extending his contract, but apologized for his comments shortly afterwards.
In his last years in office, Blair was in conflict with several members of the police board over resistance to proposed reforms as well as his resistance to cut the police service's budget. On July 30, 2014, the Toronto Police Services Board announced that it would not renew Blair's contract for a third, five-year term. He retired from the police service when his contract ended on April 25, 2015 and was succeeded by Deputy Chief Mark Saunders.
Blair declined to comment on his future plans while he was still police chief. The Liberal Party of Canada recruited Blair to be its candidate in Scarborough Southwest for the federal election held October 2015. A poll conducted by Forum Research suggested Blair would receive 39% of the vote against 29% for incumbent Dan Harris of the New Democratic Party and 27% for the Conservatives.
On April 25, 2015, Blair confirmed his intention to seek the Liberal Party nomination in Scarborough Southwest. He won the Liberal nomination on June 13, 2015.
On October 19, 2015, Blair was elected in the Scarborough Southwest riding. On December 2, 2015, Blair was named parliamentary secretary to the Minister of Justice. In January 2016, Blair was named as the head of the federal-provincial task force tasked with creating a plan for the legalization of cannabis in Canada.
On January 19, 2013, Blair was honoured by the Canadian Tamil Congress, with their inaugural “Leaders for Change Award” for his exemplary leadership during the protests of 2009 in Toronto.