Mantila started in 1992 for the first time in a World Cup competition. He finished 12th on his debut in Lahti . In the next competition in Trondheim , he placed ninth in the top ten. In the 1993/94 season he was able to place himself in the top ten several times and finished ninth in the overall World Cup . At the Nordic World Ski Championships in 1995 in Thunder Bay , he won the silver medal behind the Norwegian Fred Børre Lundberg .
In 1999 Mantila could only rarely place in the top ten in the World Cup. At the Nordic World Ski Championships in 1999 , he took 14th place in the sprint and eleventh place in the individual competition. With the Finnish team he was able to fight for the world title. His last place on the podium in a World Cup race was achieved by Mantila in 2000 in the sprint competition in Oberwiesenthal . There he took third place behind Samppa Lajunen and Bjarte Engen Vik . In the overall World Cup 1999/00 could still occupy a tenth place.
In the summer of 2000 he was able to take second place at the Grand Prix in Oberhof / Steinbach-Hallenberg and third place in Klingenthal . In the overall Grand Prix classification, he took fourth place. In the following winter season 2001 he could not repeat the achievements of the last few years and not place in the top ten in World Cup competitions. At the Nordic World Ski Championships in 2001 in Ramsau am Dachstein , he won the bronze medal with the Finnish team.
In his last active season in 2002, Mantila could only rarely place in the top twenty in the World Cup, but was still nominated for the Winter Olympics in Salt Lake City . Together with Samppa Lajunen, Hannu Manninen and Jaakko Tallus , he won the gold medal in the team competition in front of the German team. In March 2002, Jari Mantila played his last World Cup competition in Oslo .
After the sports career
After his active time as an athlete, Jari Mantila worked as a service man for the Finnish ski jumping world cup team. At the Nordic World Ski Championships in Oberstdorf in 2005 , he was arrested for drunk driving and lost his job at the Finnish Ski Association. At the end of 2005, Mantila was hired by Wolfgang Steiert as chief technician for the Russian ski jumping team.
In April 2004, Mantila and his friend Janne Ahonen founded the company Radius AY , which sells snowmobile and motorcycle accessories.