MacKenzie returned to England to sign for Third Division club Brentford in August 1964. He was mostly confined to the reserve team during his first two seasons with the club and made just three first team appearances. MacKenzie made a breakthrough of sorts during the 1966–67 season and made 20 appearances, but financialcutbacks at the stricken club forced his release at the end of the season. He made just 23 first team appearances for Brentford, but found success with the reserve team, with whom he won the London Challenge Cup in 1964–65 and 1966–67.