During his career, the 71-year-old Finnish native attempted to broker solutions to conflicts in Namibia, Sri Lanka, Iraq, Kosovo and Northern Ireland.
Ahtisaari, who has been among the frontrunners for the prize for several years, said he had not been expecting to win the 2008 award.
"But definitely I was hoping, because this is the highest recognition that a person in my profession can have," he said after the announcement.
"I think it's very rewarding to be in the same category as some people I have admired like Mandela."
Ahtisaari said the highlight of his career had been his tour as United Nations special envoy to Namibia, during which he helped guide the country to peaceful independence after more than a decade of talks.
"Of course Namibia is the most important since it took so long," he said on Friday.
Joha Christensen, a friend who worked with Ahtisaari during the Aceh peace talks, told Al Jazeera: "This is fantastic news. Of course it's meaningful for the Aceh people. I'm very happy for that."
Talking of Ahtisaari's skills as a negotiator, Christensen said: "I think he is able to provide the atmosphere where the parties in a conflict feel they are equal and can concentrate on the issues at the core of the conflict.
"In a dignified way he is the 'head master'. He brings people back to the real issues, back to reality."
Ahtisaari and Indonesia
Aceh is one of Indonesia's poorest and most underdeveloped provinces. For centuries the region was an independent sultanate while other parts of the country were colonised by the Dutch.
The Geraken Aceh Merdeka (GAM), the Free Aceh Movement declared in 1976 that the province had been illegally annexed by Indonesia in 1949. Decades of fighting followed in which thousands of people died.
Peace in Aceh had been sought numerous times, but all attempts failed before Ahtisaari succeeded. He mediated negotiations in secret talks between the Indonesian government and GAM until the conflict ended in 2005.
After studying failed peace negotiations, Ahtisaari formed the Aceh Monitoring Mission with the help of the EU and the Association of South East Asian Nations to create a space for both parties to live peacefully.