McKay was teaching at Washington Junior High in Ogden when he was elected in 1962 to the state legislature, where he served two terms. After he was defeated in a state senate race in 1966, he accepted a position as administrative assistant to Governor Calvin Rampton in March 1967. That October he was called as the Ogden LDS Stake President. He successfully ran as the Democratic Party candidate for the U.S. Congress in 1970. He was only the second freshman congressman during this century to be appointed to the powerful Appropriations Committee.
As a member of the Appropriations Committee, McKay served on three subcommittees: as chairman of the military construction subcommittee; the Washington, D.C., subcommittee; and the interior subcommittee. He saved the Intermountain Indian School from closure for a decade and helped keep the Central Utah project funded.
A confirmed Democrat, he supported Jimmy Carter's bid for reelection in 1980 and was defeated in his own bid for a sixth term in Congress in the local Reagan landslide victory. From 1981 to 1984 he served his church as a mission president in Scotland. He was unsuccessful again as the Democratic candidate for Congress in 1984 and 1986. Following his defeat in 1986, he became a consultant and lobbyist while maintaining the family farm in Huntsville.