In a state of depression, Buchanan now took a step that shook the British boxing world, and which he has regretted ever since; he sent his licence and championship rings back to the British Boxing Board of Control and, at the age of 24, announced his retirement. In the normal course of things, after 34 unbeaten contests and as the lightweight champion of Britain. the opportunities should have been infinite. Yet, so far as he could see. Buchanan was finished. He went back to carpentry, his trade as an amateur boxer, and claimed to be better off and happier, able to have a drink and not worry about his weight.
However, after his mother had passed away his father persuaded Ken to take boxing back up, and with this he went back into training. Then on 29th of January, 1970, he challenged future world Jr. Welterweight champion Miguel Velazquez in Madrid, for the European Lightweight title. cheap championship rings 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, custom championship rings 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.
Buchanan became depressed by his welcome home in Edinburgh. Putting on a sombrero to meet the expected crowd, he found a reception committee of just six, and four of those were his wife Carol and son Mark, and his parents-in-law. Edinburgh, it seemed, still did not care about its outstanding world champion. However, 10 weeks after he had won the title. Buchanan finally discovered a place where he was appreciated. In December 1970, Buchanan made his first appearance at Madison Square Garden, on the undercard of the Muhammad Ali - Oscar Bonavena fight. His opponent was Man unbeaten Canadian welterweight, Donato Paduano, who was nearly a stone heavier at the weigh-in. Buchanan realized that he would have to use all his speed and skill to overcome this dangerous opponent. and he put on a brilliant boxing master class, emerging unmarked for a decisive points win. The fans gave him a rare standing ovation and he was a favourite at the Garden from then on He finished 1970 beating Donato Panuato by a 10 round decision in a non-title bout. Americas Boxing Writers named Buchanan their 'Fighter of the Year' for 1970, ahead of the legendary Muhammad Ali and Joe Frazier after what is regarded as one of the great title fights; The Fight of the cenury. Buchanan then 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.