Born in Germany, Seissler began his career in 1960 with the KSV Hessen Kassel junior team.
In 1961, he moved to the first team which competed in the Oberliga Süd, the southern conference of the German first division back then, which preceded the creation of the Bundesliga.
[1] In 1967, he left Germany for the United States where he signed with the Pittsburgh Phantoms of the National Professional Soccer League.
With the Spurs he won the NASL championship of 1969 and Seissler played with them, in the end as captain, until they folded following the 1970 season.
[3] During his early years in the NASL, Seissler was one of the league's top forwards, being named a first team All Star in 1969, 1970 and 1971.