Born in Joliet, Illinois, Leinenweber received a B.A. from the University of Notre Dame in 1959 and a J.D. from the University of Chicago Law School in 1962. He was in private practice in Joliet, Illinois from 1962 to 1986. He was a City attorney of Joliet, Illinois from 1963 to 1967. He was a Special prosecutor, Will County, Illinois from 1968 to 1970. He was a Special counsel, Village of Bolingbrook, Illinois from 1975 to 1977. He was a Special counsel, Will County Forest Preserve, Illinois in 1977. He was a Republican member of the Illinois General Assembly from 1973 to 1983.