He was born in Fort Frances, Ontario to a blue collar family. He was a good student, but also athletically gifted and politically active, getting involved with the NDP as a teenager.
He took an undergraduate degree in philosophy and religion from Dartmouth College, which he attended on a hockey scholarship. He later obtained a degree in education from the University of Toronto and a law degree from the University of Ottawa.
He worked as a lawyer for the Canadian Labour Congress, and for the NDP government of Allan Blakeney in Saskatchewan, before being elected to Queen's Park as an MPP in 1987. During the NDP government of Premier Bob Rae, he served as Attorney General and subsequently Minister of Natural Resources.
After Rae retired from politics, Hampton became the leader of the NDP on June 22, 1996, beating Frances Lankin, who many had considered the front-runner, on the fourth ballot.
In the 1999 Ontario election, some union leaders, who have always been a crucial source of NDP support, attempted to defeat Mike Harris, the Progressive Conservative premier, by abandoning the NDP for the Liberals. This tactical voting initiative, commonly called "strategic voting," did not succeed in ousting the Harris government, but nearly decimated the NDP as they took just nine seats and 12 per cent of the popular vote in their poorest election showing since the 1950s. However, as Hampton was not judged to have been at fault, he stayed on as leader.
In Harris' second term, the government unveiled plans to privatize the public electricity utility, Ontario Hydro. Hampton quickly distinguished himself as a passionate advocate of maintaining public ownership of the utility, and published a book on the subject, Public Power, in 2003. Harris' successor as premier, Ernie Eves, was ultimately forced to back down on selling the hydro utility.
Although Hampton and the NDP were widely judged to have run an excellent campaign, they won only seven seats in the 2003 Ontario election, losing official party status. However, Hampton retained his seat and the party won a larger share of the popular vote than it had in 1999. In addition after intense lobbying to lower the minimum number of seats for party status, the resulting embarassment of the new Premier failing to follow the spirit of his promise of electoral reform compromised with additional funding for the NDP and more inquiry opportunities during Question Period
Hampton's wife, Shelley Martel, is also an MPP, representing Nickel Belt.