Pirjo Manninen first stepped into the spotlight at the 1998 Junior World Championships in St. Moritz, Switzerland , when she finished fourth over 15 kilometers and fifth over five kilometers at the age of 17. With the relay, she won the silver medal there. At the 1999 Junior World Championships in Saalfelden am Steinernen Meer, Austria, with the Finnish relay she was Junior World Champion and won the silver medal over five kilometers. After she had won a complete set of medals at the Junior World Championships in 2000 in Štrbské Pleso, Slovakia (gold in the sprint, silver over 5 kilometers and bronze over 15 kilometers), she won gold in the sprint at the 2001 World Championships . At her last Junior World Championships in 2001 in Karpacz , Poland , she was again world champion in the sprint and also in the individual race over 15 kilometers freestyle and with the Finnish 4x5 kilometer relay. Over five kilometers in classic style, she finished second behind the Swede Lina Andersson . At the 2005 World Championships she won silver in the team sprint together with Riitta-Liisa Lassila . She won her first World Cup race in Linz in December 2002 . At the 2007 World Cup in Sapporo , she was able to win the gold medal over 4 × 5 km with the Finnish relay team.