He first came to prominence in 1956 when he set up the Bund Nationaler Studenten, a far-right student organisation that was banned in 1963.
[3] He replaced Adolf von Thadden as party leader in 1971 although the battle for the leadership proved somewhat divisive as his defeated opponent, Siegfried Pöhlmann, split away from the NPD with his supporters the following year in order to establish his own group, Aktion Neue Rechte.
[5] Nonetheless, he and von Thadden did not enjoy a good relationship, due to Mussgnug's closeness to his rival Gerhard Frey.
Von Thadden left the party in 1975 over the issue and Mussgnug secured for Frey a seat on the NPD's executive committee.
[6] Succeeded by Deckert, he left the NPD after this and became involved in setting up the German League for People and Homeland (DLVH).