Robertson had played for Scotland at Schoolboy and Youth levels and for Drumchapel Amateurs before joining Forest in May 1970, making his debut for the team in October 1970. Although he was an infrequent member of the first team as a midfielder up to 1975, and was on the transfer list when Clough became manager, Robertson became a key player as a left winger under Clough and appeared in 243 consecutive games between December 1976 and December 1980. He scored the winning goal, a penalty, for Forest in the 1978 Football League Cup Final replay against Liverpool. He also scored the winner in the 1980 European Cup Final against Hamburg and provided the cross for the winning goal in the 1979 European Cup Final, scored by Trevor Francis, against Malmo FF.
Brian Clough, Robertson's manager at Nottingham Forest, was quoted as saying "John Robertson was a very unattractive young man. If one day, I felt a bit off colour, I would sit next to him. I was bloody Errol Flynn in comparison. But give him a ball and a yard of grass, and he was an artist, the Picasso of our game." In his autobiography Clough noted that "Rarely could there have been a more unlikely looking professional athlete... [He was a] scruffy, unfit, uninterested waste of time...but something told me he was worth persevering with." but that "[He] became one of the finest deliverers of a football I have ever seen – in Britain or anywhere else in the world – as fine as the Brazilians or the supremely gifted Italians."
Robertson was sold to Derby County F.C. in June 1983 on a contested transfer (the fee was set by a tribunal) that soured the relationship between Clough and his former assistant Peter Taylor, but was injured soon after joining the team and failed to reproduce the form he had shown when he played for Forest. Although he rejoined Forest on a free transfer in August 1985, he remained well below his former best and moved to non-league Corby Town F.C. at the end of the 1985/86 season. He also had stints with Stamford F.C. and Grantham Town F.C..
In 1997, FourFourTwo magazine declared that John Robertson was 63rd in the 100 greatest footballers of all time. He was also voted No 1 Nottingham Forest player of all time, forcing Stuart Pearce into second place, in a 2005 poll run by fans.
Robertson's daughter, Jessica, was born in 1983 with cerebral palsy, which left her quadriplegic and unable to speak or control her movements. She had a short life expectancy. In 1994, Robertson and his former wife Sally had challenged the hospital where Jessica was born for damages, claiming that they had caused her brain damage by a 12-hour delay to carry out a Caesarean section. However, they lost their High Court case.
Robertson released his autobiography 'Supertramp' in September 2012. He supported Rangers as a boy, but describes his time at Celtic as assistant to Martin O'Neill as the best years of his life in football.
Robertson suffered a suspected heart attack while playing tennis with former Forest team-mate Liam O’Kane on 23 August 2013. Fans of Celtic, Derby County, Leicester City and Nottingham Forest took to social network sites to wish him well.
^"The things they say: Brian Clough". FIFA. 1 May 2009. Retrieved 2009-05-01. John Robertson was a very unattractive young man. If one day I was feeling a bit off colour, I would sit next to him. I was bloody Errol Flynn compared to him. But give him a yard of grass and he was an artist. The Picasso of our game.
^pp, 152, 155, Clough, Brian (1994), Clough: The Autobiography, Partridge Press