Kasai made his World Cup debut on 17 December 1988 in Sapporo, Japan, at the age of 16, reaching 31st place.

He won after a second day of competition which was cancelled after strong winds and a crash of Christof Duffner.

At that time he was among the world's top jumpers, known for his extraordinary style, holding his body almost flat between his skis.

[2] On 29 November 2014, Kasai became the oldest World Cup winner when he shared the victory with Simon Ammann in Ruka, Finland.

He is the oldest athlete to ever perform in the FIS Ski Jumping World Cup and holds the record for the most appearances in the competition, with a total of 579 individual starts in 34 seasons between 1988–89 and 2024–25.

[12] In November 2014, Kasai became the oldest individual World Cup event winner, aged 42 years and 5 months.