Derr was appointed to the Maryland Senate to represent District 3 in 1983, which covers parts of
Frederick and
Washington Counties, following the death of incumbent state senator Edward P. Thomas Jr. In 1986, Derr was elected to a full term, defeating Democratic challenger Gerald Downs in the general election with 68 percent of the vote. In 1990, Senator Derr was not challenged and received 100% of the vote. In the 1994 primary election, Derr received more competition, this time from Republican Charles R. Luttrell. Luttrell managed to garner 47% of the vote. Derr went on to win the general election, this time, oddly enough, beating the same candidate, Charles R. Luttrell, who apparently switched parties to challenge Derr again. In 1998, Derr saw the end of his senatorial career. He was defeated by Republican challenger
Alex X. Mooney in the Republican primary election. Mooney received a sizable majority of the votes, 63% to Derr's 37%. Mooney went on to win the general election, defeating the Democrat Ronald S. Bird. ==Education==