Mason was selected first overall by the expansion Minnesota Vikings in the 1961 NFL Draft. In six seasons with the Vikings, he rushed for 3,252 yards and scored 28 touchdowns. In 1967, he was signed by the Los Angeles Rams. He played with the Rams for four years, but accumulated only four touchdowns and less than 900 yards. He finished his career with the Washington Redskins in 1971 before retiring.