Horst Hörnlein started his career in 1964, as a junior with Reinhard Bredow. Hörnlein then rose to the seniors and slid with Rolf Fuchs at the 1965 World Championships at Davos, placing third. In 1966, he re-united with Bredow but they were disqualified at the Worlds by the Polish sports administrator Lucjan Swiderski because they had illegally heated their runners. Nevertheless Hörnlein and Bredow went on to win the 1972 Winter Olympics at Sapporo, place second at the 1969 World Championships, win the 1970 and 1972 European Championships and the 1973 World Championships. In the singles Hörnlein was also 1971 European Champion, after placing third in 1970.
After his own career he became a coach to bobsledders, first for East Germany (1973-90), and later for the British. His pupils Mark Tout and Sean Olsson have won several titles. He has also worked as a coordinator and trainer for underdeveloped bobsledding countries within the Fédération Internationale de Bobsleigh et de Tobogganing (FIBT).