Bethea was selected by the Cowboys in the first round of the 1978 NFL draft, after playing college football for the Michigan State Spartans and winning Big Ten Most Valuable Player in 1977.
[2] He accepted a football scholarship from Michigan State University, where he began as a tight end until being converted into a defensive tackle during his sophomore season.
[3] Bethea was selected by the Dallas Cowboys in the first round (28th overall) of the 1978 NFL draft, who were looking for an eventual successor to either Harvey Martin or Ed "Too Tall" Jones.
His most notable play was stopping Los Angeles Rams' fullback Jim Jodat at the Cowboys' 11 yard line on fourth and one during the third quarter of the 1978 NFC Championship game.
[5] In 1979, after Jones retired to pursue a boxing career, Bethea was named the starter at left defensive end at the beginning of training camp.
He started in the NFC Championship Game against the San Francisco 49ers in place of an injured John Dutton, and although he recovered a fumble, he was one of the defenders that Joe Montana released his pass over so Dwight Clark could make "The Catch", propelling the 49ers to a 28–27 win.