British Prime Minister Tony Blair attended the ceremony where Protestant Democratic Democratic Unionist Party leader Ian Paisley, 80, was sworn in as first minister and Martin McGuinness, 56, of Sinn Fein became deputy first minister, the BBC reported.
The British government suspended self-rule in October 2002 when allegations of intelligence gathering within the Stormont parliament came to light.
In March, DUP leader Ian Paisley met with Sinn Fein president Gerry Adams and the two agreed to share power to end London rule.
Before the signing ceremony, Paisley told reporters he was optimistic about the arrangement.
"Today we are starting upon the road which I believe will take us to lasting peace in our province," he said. "From the depths of my heart I can say I believe Northern Ireland has come to a time of peace, a time when hate will no longer rule."