In 1959, McPeak joined the Washington Redskins as an assistant under head coach Mike Nixon. After Nixon's dismissal following the 1960 NFL season, McPeak was promoted to head coach, and would remain in that position until 1965. Although the Redskins did not have a winning season under McPeak (he had an overall 21-46-3 record as head coach), it was during his tenure that the team would acquire players (many of whom would become Hall-of-Famers) that would eventually play a part in their later winning years such as Sonny Jurgensen and Bobby Mitchell, and draft future stars such as wide receiver Charley Taylor, tight end Jerry Smith, center Len Hauss, and linebacker Chris Hanburger.
In 1967, McPeak would join the Detroit Lions as an assistant coach, a position he held until 1972 when he moved on to the Miami Dolphins to replace Howard Schnellenberger who became head coach of the Baltimore Colts. Unfortunately he would only be an assistant in Miami for two seasons because he would suffer a stroke, and would take several years to recover. After his recovery, he would join the New England Patriots where he became director of scouting for a dozen years.