The 43-year-old Kevin Reynolds from Scotland, who was described as a homeless "fighting drunk" in court, had pleaded guilty to assault and to attacking and racially abusing a police support officer who was called to the scene.
Ferguson was waiting to be picked up outside the Euston train station in central London after traveling from Manchester to attend an annual charity event when the attack happened on Sept. 10. He had soreness and tenderness after being punched, but was not seriously injured.
Reynolds had been released from a prison sentence for assaulting a police officer just five days before the attack on Ferguson.
He was sentenced to 15 months jail for the attack on the support officer, with a concurrent five-month sentence for the assault on Ferguson.
"You are, in the words of the probation officer, a homeless alcoholic who accepts no responsibility for your violent behavior, and indeed minimizes the seriousness of your offending," Judge Paul Dodgson of Southwark Crown Court told Reynolds in sentencing.
Ferguson had thought the man staggering toward him was a beggar. Instead, the court heard that Reynolds punched him and said: "I'm sorry Fergie, I did not know it was you."
"He really gave me a whack. At the time my natural instinct was to react and defend myself," Ferguson said in a police statement. "However, I decided against it as I did not know if he was carrying any weapon and might come at me again.
"This is not something I expect to happen to me when I am going about my daily business, and it completely stunned me."
The City of London magistrate Daphne Wickham, who heard the case last month, described Reynolds as a "fighting drunk" after he had consumed half a bottle of vodka and several beers before the attack.
Ferguson, who is also Scottish, has won 20 titles in two decades as manager of Manchester United, including nine Premier League titles and the 1999 European Cup. He was knighted by Queen Elizabeth II in 1999 for services to soccer, which allows him to be called Sir Alex.