A court heard Jack Smith 'misread' the situation and turned violent when he saw a man trying to help his girlfriend after she collapsed in the street.
Smith, 29, approached the victim as he had his arm around the woman and kicked him in the head.
Witnesses described how Smith followed up the attack with another kick to the victim's head as he lay prone on the ground.
Leeds Crown Court was told the second strike to the head was like "taking a penalty kick."
Smith was jailed for 20 months after pleading guilty to causing grievous bodily harm.
Andrew Horton, prosecuting, said the attack happened in a street in Horsforth in the early hours of April 6 this year.
Smith and his fiancee had been on a night out with another couple and he had taken the Class A drug MDMA.
The prosecutor said it is believed the two women had their drinks spiked by others during the evening and began to feel unwell.
Smith's partner was crying by the kerbside in the street when the victim, who was returning home from a night out in Leeds city centre, approached her to try to help.
Smith carried out the attack when he saw the 38-year-old man with his partner.
Mr Horton said: "He made no attempt to find out what was going on and punched the complainant once to the face and kicked him in the head."
A resident in the street watched the incident from his home and ran outside to intervene.
The witness said he then saw Smith line up another kick after the victim had gone to the ground "like a rag doll."
He described the second kick as being "like taking a penalty shot."
The victim was left unconscious from the attack.
He suffered complex fractures to his facial bones and had to undergo surgery to have metal plates fitted to his face.
Smith, of Crowhill Avenue, Cleethorpes, was arrested and told police in interview that he had misread the situation.
He said he had been drinking gin and tonics and had taken MDMA before the attack.
The defendant told officers he was sorry for what he had done.
He pleaded guilty to inflicting grievous bodily harm.
Nick Worsley, mitigating, said Smith had no previous convictions and faced losing his job and his home if he was sent to custody.
He said: "He thought the complainant had done something to his girlfriend."
Mr Worsley added: "He will never, ever, appear before the court again."
Jailing Smith, The Recorder of Leeds, Judge Guy Kearl, QC, said: "He was senseless on the floor and he was no threat to you, and yet you followed up your attack on a man who was defenceless on the floor with a kick to the head.
"It was like a penalty kick.
"It was with force, and the injuries indicate it was with force."
The judge added: "You were attacking a Good Samaritan. Not once, but twice, when he was defenceless.
"To kick a man to the head when he was defenceless on the ground cannot be overlooked."