He was part of the Norwegian winning team in relay at the World Orienteering Championships in 1981 and 1983. He was awarded Bjarne Bjerckes pokal for his results in the long distance national championships. Thon was running the last (fourth) leg, and started 1.17 minutes behind leading Switzerland.
Thon became Norwegian orienteering champion in relay with IF Sturla seven years in a row, in 1979, 1980, 1981, 1982, 1983, 1984, and 1985. He became Norwegian champion in the long distance in 1979 (course length 233 kilometers, 22 controls), and again in 1980 (course length 270 kilometers, 25 controls). He won an individual silver medal in the Norwegian championships in 1982 (course length 121 kilometers), behind Egil Iversen, and an individual bronze medal in 1983 (course length 133 kilometers). Thon competed at the 1981 World Orienteering Championships in Thun, Switzerland, where he placed 10th in the individual competition, and won a gold medal in the relay event, together with Øyvin Thon, Tore Sagvolden and Sigurd Dæhli. At the 1983 World Orienteering Championships in Zalaegerszeg, Hungary, he won his second world championship relay, together with Morten Berglia, Øyvin Thon and Tore Sagvolden. He passed the leader when the Swiss runner had a miss at the tenth control, and won the relay with a margin of one and a half minute ahead of the Czechoslovak team Harald Thon is older brother of Øyvin Thon, a seven-times world champion in orienteering.
