For the 1963 NFL Draft, Buchanan was selected 265th overall in the 19th round by the NFL champion Green Bay Packers. The 6 ft 7 in (2.01 m) 287 lb (130 kg) Buchanan was the first overall selection in the AFL Draft by the Kansas City Chiefs. Eddie Robinson, his coach at Grambling State, where he had been an NAIA All-American in 1962, called him "the finest lineman I have seen." Buchanan was the first black number one draft choice in Professional Football. He came from a small historically black university.
Others who had watched Buchanan in action were equally enthusiastic. Buchanan had the physical size plus the athletic instincts to be exceptionally successful at his job of foiling opposing offenses. He was particularly effective at intimidating the passer and in one season alone (1967) he batted down 16 passes at or behind the line of scrimmage. He was clocked at 4.9 in the 40-yard dash and 10.2 in the 100-yard dash at Grambling State and with that speed he could range from sideline to sideline to make tackles.
In spite of the weekly pounding he took on the line of scrimmage, Buchanan was extremely durable. He played in 182 career games that included a string of 166 straight. After dabbling briefly at defensive end as a rookie, Buchanan settled down to his permanent job as the Chiefs' defensive right tackle. He was named to his first AFL All-Star Game after his second season and played in six AFL All-Star games and two AFC-NFCPro Bowls.
In 1999, he was ranked number 67 on The Sporting News' list of the 100 Greatest Football Players, right behind his former Chiefs teammate Bobby Bell at number 66. The Chiefs also retired his uniform number 86.
After his football career Buck started the construction company All Pro Construction, Inc. in Olathe, KS. They specialized in interior renovations and miscellaneous bridge work. Buchanan's widow, Georgia continued to run the company after his death until 2006 when she sold.