He started boxing professionally on 20 September 1965, beating Brian Tonks by a knockout in the second round in London.

[2] Buchanan extended his winning streak to 23 consecutive bouts before challenging Maurice Cullen on 19 February 1968 for the British lightweight title in London.

[2][3] 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 29 January 1970, he found his first stone on the boxing road when he challenged future WBC junior welterweight champion Miguel Velasquez 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 defence of the British lightweight title.

Many experts believed 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.

Buchanan defeated former world junior welterweight champion Carlos Morocho Hernández by knockout in round eight of a non-title bout, in Wembley on 11 May 1971.

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 on 29 April 1972.

[16] His next defence came on 26 June 1972, against Panama's greatest, the then undefeated Roberto Durán at the Madison Square Garden (MSG) in New York, in a bout which had a highly controversial ending.

[17] The New York Times columnist Red Smith wrote that LoBianco had to award the victory to Durán, even if the punch was a low blow, as "anything short of pulling a knife is regarded indulgently" in American boxing.

[18] In his next fight, Buchanan beat former three-time world champion Carlos Ortiz by a knockout in six, also at Madison Square Garden on 20 September 1972.

[21] Once again Durán broke the agreement, and subsequently had his licence suspended by the New York State Athletic Commission on 4 April 1973.

[21] The New York State Athletic Commission had been attempting for two years (1972–1974) to get Durán to honour an agreement to fight Buchanan.

[2] Buchanan re-grouped once again, and won in a defence of the European lightweight title against Giancarlo Usai by a knockout in 12.