Buchanan was born in Edinburgh, Scotland. He started boxing professionally on September 20, 1965, beating Brian Tonks by a knockout in two rounds in London. He spent much of the early parts of his career fighting undistinguished opponents in England. His Scottish debut came in his 17th fight, when he outpointed John McMillan over 10 rounds on January 23, 1967. Prior to that, he had also beaten Ivan Whiter by a decision in 8 rounds.
Buchanan ran his winning streak to 23 consecutive bouts before challenging Maurice Cullen on February 19,1968 for the British Lightweight title in London. He knocked Cullen out in round 11 and became a world classified Lightweight challenger.
He continued his way up the world Lightweight rankings by defeating Leonard Tavarez, Angel Robinson Garcia and Whiter (in a rematch) among others, but on January 29, 1970, he found his first stone on the boxing road when he challenged future world Jr. Welterweight champion Miguel Velazquez in Madrid, for the European Lightweight title. Buchanan lost a 15 round decision to Velazquez, but nevertheless, he continued his ascent towards the number one spot in the rankings by beating Tavarez in a rematch, Chris Fernandez and Brian Hudson, the latter of whom was beaten by a knockout in five in a defense of the British Lightweight title.
On September of that year, Buchanan travelled to Puerto Rico, where he would meet Ismael Laguna, the world Lightweight champion, on September 26. Many experts thought that San Juan's warm weather would affect Buchanan, but he upset those who thought that way and beat Laguna by a 15 round decision to become world's Lightweight champion. At that time, the WBA and the British Boxing Board of Commisioners (BBBC), were in the middle of a feud, and Buchanan was not allowed to fight in the United Kingdom. He had to resort to fighting overseas for a short period of time.
He finished 1970 beating Donato Panuato by a 10 round decision in a non-title bout, and then he began 1971 by going to Los Angeles, where he retained his title with a 15 round decision over Ruben Navarro. After that, he was allowed to fight in the United Kingdom again, and he returned there to beat former world champion Carlos Morocho Hernandez by a knockout in eight.
Then, he flew to New York to meet Laguna again, this time defending his world title. Buchanan retained the title with another decision over Laguna, and then he had a couple of non-title affairs, one in London and one in South Africa. The South African fight against Andries Steyn in Johannesburg was a mismatch with his opponent's corner throwing in the towel in the third round.
He was stripped of the WBC title for failing to defend against Pedro Carrasco, but he remained the WBA world Lightweight champion. His next defense came on June 26 of 1972, against then undefeated Roberto Duran at the Madison Square Garden (MSG) in New York. This bout proved to be one of the most controversial in boxing history. During an exchange to end round 13, Duran landed a blow to the mid-section that might have struck Buchanan in an illegal area of the body. Buchanan has always claimed that the blow was low, and replays, which have been shown countless times on TV, are inconclusive. Referee John LoBianco said, however, that he thought the blow was legal, and therefore, the bout, and the world championship, were given to Duran by a technical knockout in 13 rounds. Buchanan required hospitalization and surgery after the bout.
In his next fight, Buchanan beat former 3 time world champion Carlos Ortiz by a knockout in six, also at the MSG, and he finished 1972 with a win over Chang Kil Lee.
In 1973, Buchanan started out by beating future world Lightweight champion Jim Watt by a decision in 15, to regain the British lightweight title. Soon, he embarked on another international tour that included more fights in the United States, several fights in Denmark, and one fight in Canada. He won each of those fights, leading towards a challenge of European Lightweight champion Antonio Puddu in Italy, and Buchanan added the European Lightweight championship belt to his shelf by defeating Puddu by a decision in 15 rounds. He retained the title by beating Tavarez for the third time, this time by a knockout in 14 at Paris, and then he travelled to Japan to fight for the world title again. This time, however, he was defeated by a decision in 15 rounds by the WBC's world champion, Ishimatsu 'Guts' Suzuki.
Buchanan re-grouped once again, and won in a defense of the European Lightweight title against Giancarlo Usai by a knockout in 12. But he retired from 1976 to 1978, leaving the European Lightweight title vacant.
When he returned to professional boxing in 1978, he won two straight bouts, but everything else started going backwards for him. Challenging Charlie Nash in Copenhagen, he lost by a decision in twelve. In 1980, he won two bouts in a row, but after that, he lost five bouts in a row, finally retiring for good after losing to George Feeney by a decision in eight on January 25 of 1982.
He retired with a record of 61 wins and 8 losses in 69 professional bouts, with 27 wins by knockout.