Prohaska was born into a working class family. His father was a worker and his mother cleaned rooms. His father was coach of a youth team of Vorwärts XI. Here Prohaska started his career at the age of 9, then he played for the minor league club SC Ostbahn XI in Vienna. Then he went on to FK Austria Wien and became professional player. He played at the age of 16 for the first team of Austria Wien. Till 1980 he won with the team 4 Austrian league titles and 3 Austrian cup wins. 1980 he went to Italy where he played for Inter Mailand and later for AS Roma. In Italy he won the championship with Inter and the cup with Roma. From 1983 to 1989 he played again for FK Austria Wien and won another 3 championships and one cup. 1989 he retired from playing football.