During his youth in Kiel, Sieloff aimed to become a boxer, having stood in the ring 25 times by the age of 14. When his family moved to Rottweil near Stuttgart and after visiting football games of VfB Stuttgart at Neckarstadion, Sieloff's interest however shifted from boxing to football. In October 1960, he played his first game for VfB Stuttgart in the Oberliga Süd. There he soon became a fixture in the stopper position in Stuttgart's team.
Physically strong yet technicall adept, Sieloff soon got the attention of German national team coach Sepp Herberger, who first called him up for a 1964 friendly against Finland in Helsinki. During the next year, Sieloff became the standard sweeper of Germany. As a player, Sieloff became known for surging forward from the sweeper position, often scoring with long range shots.
Unfortunately for Sieloff, he lost his starting place in the German national team during the 1965-66 season and thus did not feature in the 1966 FIFA World Cup. During the next four years, the sweeper position was usually that of either Willi Schulz or Karl-Heinz Schnellinger and thus Sieloff did not gain anymore caps up to 1970. After the 1970 FIFA World Cup, with Willi Schulz retiring from international play and Karl-Heinz Schnellinger not featuring regularly in the German lineups due to playing abroad, Sieloff revived his international career, which was also fostered by his success at club level with Borussia Mönchengladbach.
In 1969, Sieloff had made the change from VfB Stuttgart to Borussia Mönchengladbach. Their coach Hennes Weisweiler intended to strengthen the defense of his young team, which was already very strong offensively, but lacked the defensive thoroughness to succeed in a long league campaign. For the 1969-70 season, Mönchengladbach added Sieloff as sweeper as well as Ludwig Müller (footballer) as the stopper. With the defense strengthened, Borussia Mönchengladbach won the 1969-70 and the 1970-71 Bundesliga seasons.
With Sieloff being one of the celebrated stars of the team, he made his comeback for Germany in April 1970 in a friendly against Romania. After the 1970 World Cup, with Schulz retired, Sieloff once again became the starting sweeper of Germany. However during 1971 it became apparent that Franz Beckenbauer, who previously played in midfield, was the strongest option at the sweeper position, which meant the end of Sieloff's international career.
For Borussia Mönchengladbach, Sieloff played until 1974 when he moved to Alemannia Aachen, where he remained until 1976. After his career, Sieloff became chief of the Mercedes Benz company-facilitated sports activities group.