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. Whilst leading this group he became involved with the Deutsche Reichspartei, becoming deputy chair of their Heidelberg group and following the party into the NPD in 1964. By 1968 he had been appointed chair of the NPD in Baden-Württemberg and was elected to the StuttgartLandtag that same year (until 1972). He replaced Adolf von Thadden as party leader in 1971 and followed a largely similar course in party policy terms, in the process becoming the party's longest serving leader to date. Challenged from time to time by Günter Deckert, who advocated a more confrontational style of politics than Mussgnug, he was finally defeated by his challenger in 1991. He left the NPD after this and became involved in setting up the German League for People and Homeland (DLVH). He disappeared from politics when this group proved unsuccessful.