Fahnhorst was used off of the bench and Special Teams for his first two years before he was elevated to their starting Right Tackle during the 1976 season. The native Minnesotan became one of the more unheralded Niners lore players, as he was a steady presence for years. In 1984, he was arguably the best Lineman in the NFL, as shown by his 18 in Approximate Value and First Team All-Pro. San Francisco won the Super Bowl in the 1981 and 1984 season, and Fahnhorst was a huge reason why they won those championships.
The Tackle would suffer a nasty neck injury in 1987, and he would retire after the season's end.