Hemmi was a member of the Swiss national team from 1969. He won his first points in a World Cup race on January 11, 1970 when he finished fifth in the Lauberhorn Slalom in Wengen . Nine days later he improved to fourth place in the giant slalom in Kranjska Gora . In the 1970/71 and 1971/72 seasons there was only one countable result. For this reason, he also missed qualifying for the 1972 Winter Olympics . In 1973 and 1974 he could never classify himself in the points. Often Hemmi, who was forced to drive more risky due to his small stature, fell far behind in the second run or was eliminated, which is why he was nicknamed "Sturzenegger". However, in 1974 he won the first of a total of five Swiss championship titles.
A clear upward trend was noticeable from the 1974/75 season . Hemmi achieved five results among the top ten and stood on March 2, 1975 as second in the giant slalom on Mount Garibaldi for the first time on the podium. Although he again had three top 10 results at the beginning of the 1975/76 season , Hemmi was not one of the favorites before the 1976 Winter Olympics . At the Axamer Lizum near Innsbruck , however, he surprisingly won the gold medal in the giant slalom, ahead of Ernst Good and Ingemar Stenmark ; after the first run he was still in third place. The Olympic victory was also considered a world championship title. Hemmi was able to confirm this success on March 18, 1976 at Mont Sainte-Anne with his first World Cup victory . In addition, he was voted Sportsman of the Year in Switzerland.
In the 1976/77 season , Hemmi drove at a consistently high level. He won three more World Cup giant slaloms: in Val-d'Isère , in Ebnat-Kappel and at Chuenisbärgli in Adelboden . There were also two second places and a third place. He decided the giant slalom discipline ranking for himself; like Stenmark, he had achieved 115 points, but the higher number of strike results was decisive for him to win this "little crystal ball" alone. In the overall standings, he took seventh place. Hemmi was less successful in the winter of 1977/78 ; at the end of the season there were three second and fourth places. He also achieved fourth place in the giant slalom at the 1978 World Cup in Garmisch-Partenkirchen . The 1978/79 season was similar , in which Hemmi again failed to celebrate a victory, but came in second and third and fourth three times. At the end of the season he announced his retirement from top-class sport.
The trained bricklayer Hemmi founded the trading company Heval in 1984 (the company name is made up of the surname and his place of residence Valbella ). The company sells barriers, safety nets, gate poles and other accessories for holding ski races. In 2007 he sold it to Michael Bont, who until then had been the head coach of the Finnish women's national team. Today, Hemmi presides over the World Cup Lenzerheide association , which is responsible for organizing the World Cup races in Lenzerheide .