Hefti contested the first race of his career as a brakeman in the two and four man of Marcel Rohner. With this in 1999 he was runner-up four-man at Cortina d' Ampezzo and in the season 1999/2000 overall World Cup champion in this discipline in the year. After he was changed to Martin Annen and had won the World Cup 2000/ 01 the overall World Cup title winner in the two-man bob, he won at the 2001 World Championships in St. Moritz together with Annen the bronze medal. The following winter 2001/ 02 was for the team the most successful so far. In addition to winning the bronze medal in the two-man at the 2002 Winter Olympics in Salt Lake City triumphed in both the overall World Cup, so both two - and four-man bobsleigh in. It was followed by two seasons with no greater success before it thanks to consistently good results in the Bob World Cup 2004/2005 again won the title in two-man bob. Also at the World Cup, which was held in Calgary in 2005, she reached the bronze medal. Also the bronze medal, even in both disciplines, Hefti won together with Annen at the Olympic bobsleigh competitions in Turin in 2006
After Martin Annen End of career in 2007, Hefti worked as a brakeman in Ivo Rüegg's team. Although he never went there in World Cup races at the start, he scored his biggest success to date, winning the gold medal in the four-man event at the Bob World Championships 2007 in St. Moritz with him. Prior to the 2007/ 08 season, he formed his own team, both in two - and four-man bobs in, with whom he initially launched in the European Cup. For the 2008/09 season he qualified with Thomas Lamparter in the team for the World Cup and won there 29 November 2008 in Winterberg his first World Cup race in the two-man bob. Also in the other two-man race of the season Hefti well placed, while the Viererbobteam in the first competitions always missed the Top 10. At the end of the season, he finished in the two- man and four the first and seventh place in the overall World Cup standings. In the next season he was able to win four races in the two-, finished second in the overall standings at the end but only the fourth place. In the four-man it was even enough only to 16th place in the overall World Cup standings. His biggest success as a pilot Hefti secured by winning the gold medal in the two-man bob at the 2010 European Championships in Igls. At the 2010 Winter Olympics in Vancouver he could due to a concussion that he had suffered in a practice crash, do not participate in the race in the two-man bob. In the four-man event he participated as a brakeman in the team of Ivo Rüegg, which came in sixth.