Portmann, a 6'11'' center, graduated from Sheboygan North High School in Sheboygan, Wisconsin in 1986. He averaged 15 points, 9.2 rebounds and 5 blocks as a senior[2] and was named Wisconsin's Mr. Basketball by the Wisconsin Basketball Coaches Association in 1986.[3] He was recruited by schools including Marquette, Michigan, Missouri and Wisconsin. Portmann chose the University of Wisconsin.[2] As a member of the Wisconsin Badgers men's basketball team, he made 118 appearances (30 starts) between 1986 and 1990, averaging 4.3 points, 3.5 rebounds and .9 blocks per contest.[4] Portmann ranked second all-time in blocked shots (102) behind Brad Sellers, when he graduated in 1990.[5]
In the professional ranks, Portmann played for the CBA's Wichita Falls Texans from 1990 to 1993. His CBA career high were 11.2 points per game in the 1991–92 season.[6] He played for Racing Basket Mechelen in Belgium in 1993–94, for KK Split in Croatia in 1994-95[1] and for SV Oberelchingen in Germany in 1995–96.[7] Portmann represented the Quad City Thunder in 1995-96 and 1996–97. He made the CBA All-Defensive Team in 1991-92 and 1995–96.[6] Portmann also earned All-Star honors while competing in the CBA.[8]
He played for the Toshiba Red Thunders in Japan.[9] In 1999-2000, Portmann was a member of the SeaHorses Mikawa in the same country.[10]
Portmann started working as Executive Associate Athletic Director for Compliance at Midwestern State University.[11] His son Max played college basketball at the University of Maryland, Baltimore County.[12]