Frommelt is a son of Christof Frommelt, a cross-country skier and the Olympic flag bearer for Liechtenstein at the 1948 Games. At his first Olympics, in 1972, he competed in the downhill, slalom and giant slalom, but with little success. Two years later he won the first alpine skiing medal for Liechtenstein at world championships, a bronze in the downhill, and in 1978 he won another bronze, in the giant slalom. In 1976, he received a world championships silver for his combined results in the slalom and downhill at the 1976 Olympics – world championships were then combined with Olympics in the Olympic years, but the combined results did not count for the Olympics. During his career, Frommelt had only one podium finish in the World Cup events.
After retiring from competitions Frommelt graduated in business management and worked as a financial planner for the Liechtenstein National Bank. He was also active in politics with the centre-right Fortschrittliche Bürgerpartei and served a term as a municipal councillor.
His brother Paul won another Olympic bronze medal for Liechtenstein, in 1988. His another brother, Peter, was international table tennis competitor.