Seibold also won four medals at the ICF Canoe Sprint World Championships with a gold (K-2 1000 m: 1970), two silvers (K-2 1000 m: 1971, K-4 1000 m: 1966), and a bronze (K-2 500 m: 1970).

The first serious success at the adult international level was achieved in 1966, when he was included in the main squad of the Austrian national team and visited the World Championships in East Berlin, from where he brought the silver medal, won in the four-man kayak competition at a distance of 1000 meters.

A year later, he competed at the European Championships in Duisburg, West Germany, where he won the bronze medal among the fours in the ten kilometers.

Thanks to a series of successful performances he has got the right to protect honor of the country at the summer Olympics 1968 in Mexico city — in twos on a thousand metres along with partner Günther Pfaff won in the final the third place, having conceded only to teams from the USSR and Hungary, and thus won the bronze medal, while in the four-man competition which in addition to Pfaff also includes rowers Helmut Hediger and Kurt Lindegren showed in the finals only the seventh result.

The following season, at the World Championships in Belgrade, Yugoslavia, he tried to defend the title of champion in the kilometer discipline of two-man kayaks, but this time he and Pfaff finished second and had to settle for silver awards — they were ahead of the German crew of Rainer Kurt and Alexander Slatnov.