Wells's big breakthrough came at the start of the 1978 season, when his times and victories began to improve, and he won the U.K. 100/200 Championships. British sprinters had made little impression on the international scene, and the sight of the Scot winning two gold medals (200 m, 4 x 100 m), and a silver (100 m) at the Commonwealth Games in Edmonton, Alberta, Canada, was a surprise for British athletics fans.
At the start of the 1980 season, Wells won the AAA's 100 metres, then went to Cote D'Azur to finish preparing for the 1980 Moscow Olympic Games. Wells never used starting blocks, until a rule change forced him to do so for the Moscow Olympics. In Moscow, Wells qualified for the final, with a new British record 10.11 s, where he faced pre-race favourite Silvio Leonard of Cuba. In a photo finish, both Wells and Leonard set a final time of 10.25 s, but Wells became the oldest Olympic 100 m champion at that time. The 200 m final was another close affair, but this time Wells won the silver medal behind Pietro Mennea, who beat him by 0.02 s; again he set a British record of 20.21 s. He broke a third British record, 38.62 s, with the sprint relay team that finished fourth in the final. Following the Moscow Olympics, there was a general feeling that Wells's gold medal had been devalued by the boycott of the games; however, he silenced the doubters by beating all comers after the games. At the end of 1980, Wells was also awarded Scottish sports personality of the year.
In 1981, after a successful tour of Australia and New Zealand, Wells won the European Cup 100 metres, beating the new East German sprint sensation Frank Emmelmann. Wells also finished 2nd in the 200. He then demonstrated his calibre by finishing first in the "IAAF Golden sprints" in Berlin (the most prestigious sprint event on the athletics calendar); although defeated by the Frenchman Hermann Panzo in the 100, he dominated the top four American sprinters in the 100/200, Wells winning in an aggregate 30.30. To add to this, he won the 100 metres at the "IAAF World cup" in Rome, beating a young Carl Lewis; he then finished 2nd in the world cup 200. Afterwards, he beat the top Americans again, when he won a 200 in Belgium.
In 1982, in Brisbane, Queensland, Australia, Wells won two more Commonwealth titles in the 100 m and 200 m, and a bronze medal in the relay. He shared the 200 m title with Mike Macfarlane of England in a rare dead heat.
In 1983, he won his 3rd European Cup title by winning the 200 metres, beating his old adversary Pietro Mennea in London, and again took 2nd in the 100. He then went on and finished a respectable 4th in both the 100/200 sprint finals at the "IAAF World Championships" in Helsinki; although not picking up a medal, he still beat some top scalps, and could still mix it with the world's best. In 1984, his second Olympic appearance ended with the 100 m semi-finals, but he was a member of the relay team that finished 7th in the final.
Wells missed most of 1985 with injury. He was not selected for the Commonwealth Games in Edinburgh in 1986, as he had failed to compete at the Scottish trials. However, in a stunning comeback, he soundly beat both Johnson and Mahorn, the respective Commonwealth 100 m and 200 m champions, at Gateshead. Wells was consequently selected for Stuttgart in the European championships, and placed a solid fifth in both the 100 m and 200 m finals. To add to this, he also had a victory against Linford Christie at Crystal Palace at the end of 1986. One of his last victories was winning the Inverness Highland Games 100/200 double in 1987.
Wells's later career was punctuated by injury, but by competing into his mid-30s he helped to set the trend for sprinters to have longer careers.
After retirement, he was a coach for the British bobsleigh team. His wife Margot Wells was also a Scottish 100/100 hurdles champion, and they are now based at Guildford, Surrey where she is a fitness consultant, and Allan is a systems engineer. Wells was also inducted alongside Eric Liddell and Wyndham Halswelle (two other former Scottish Athletic Olympic Champions) into the Scottish Sports Hall of Fame. Allan currently coaches the Bank of Scotland specialist sprint squad alongside another former Scottish sprinter, Ian Mackie. Wells`s P.B. for the 100 metres is 10.11, and for the 200 metres is 20.21, which he did in the Moscow 1980 games, and both are still Scottish records to this day.
Wells featured in the video for Belle & Sebastian's song "I'm A Cuckoo".
In the mid-1980s, Allan was one of the first to be seen sporting cycling shorts, a precursor to the now omnipresent Lycra running shorts. The sight of these led to him being dubbed `Wilson of the Wizard` (an athletic comic book character from a bygone age).
Wells was the last white man to win an Olympic 100m final. Indeed 1980 was last time a white athlete made it to the men's 100m final.