Leithe, who started for Moen SK, made his first international appearance at the 1998 Nordic Junior World Ski Championships in Pontresina . There he won the silver medal with the relay and finished 11th over 30 km classic. He ran his first World Cup race in March 1999 in Oslo , which he finished in 62nd place over 50 km freestyle. In the following year he got his only World Cup point with the 30th place in the sprint. In November 2000 he completed his fourth and thus last World Cup individual race in Beitostølen , which he finished in 45th place over 10 km freestyle. He also achieved his only podium finish in the World Cup with third place in the relay.