Manuela Stellmach started in 1988 for East Germany and in 1992 for the reunified German team. As part of the GDR doping investigations in 1998 it was announced that she had received anabolic steroids from her coach Volker Frischke, but she was also one of the few GDR female swimmers who was successful after German reunification.
Stellmach started her international career at the 1985 European Championships, winning two gold and two silver medals. At the 1987 Europeans she added three gold and two silver medals, and in 1989 three more golds and one silver medal. Swimming for Germany, she completed her European collection with one silver in 1991 and a gold in 1993. Stellmach also took part in two World Championships, in 1986 for East Germany, earning two golds and one silver medal, and in 1991 earning a full medal set. At the Olympics she won two relay gold medals in 1988, adding a bronze that year in the 200 free and another relay bronze in 1992 for the reunified Germany. Stellmach retired after the 1994 season.