On the local level Foss was a deputy member of Oslo city council from 1971 to 1975.
From 1973 to 1977 he was the leader of the Young Conservatives (Unge Høyre), the youth wing of the Conservative Party. Per Kristian Foss was mentioned as a possible new leader for the Conservative Party after Jan Petersen resigned in 2004, but he declined to run and instead supported Erna Solberg. After Høyre struggled in elections and polls during the first years of Erna Solberg´s leadership he was also mentioned as a possible new leader, but he never challenged Solberg for the leadership position. At present Foss is deputy chairman of the Conservative Party and member of the party's central board.