Gurvinder Toor, from Tipton, attacked Taranjeet Singh as they arrived at Mr Singh's home in Kempsey Close, Oldbury, on September 20, 2020.
Mr Singh, 32 and known as Taran, had pulled up outside the property when he was attacked by Toor, 27, who was sitting in the rear passenger seat.
The victim suffered nine wounds to his arm and neck during the incident, which happened at around 9pm as they returned from buying food from a takeaway, Wolverhampton Crown Court heard.
Mr Paul Spratt, prosecuting, told the jury that three wounds to a key artery resulted in Mr Singh suffering left side paralysis, paralysis to his vocal cords and a brain injury – and he would never completely recover from those injuries.
Mr Spratt added Toor, known as Gura, was arrested at the scene outside the family's home. The knife used to inflict the injuries was also recovered at the scene, the court was told.
The defendant told the officers that he had been drinking and that he taken medication for his mental health illness, but he had not taken them for about three days prior to the incident taking place, a judge heard.
Mr Spratt added Toor told the officers that while he was in the car he had thoughts that Mr Singh’s young family was in danger of being harmed by him.
Giving evidence under oath, Sundeep Uppal said a number of people were also at the property including their children when her husband came inside and said he had been struck by Toor.
Mrs Uppal said: “When I first saw him I said ‘What happened to you?’ And he said, ‘Gura’s hit me’. I said, ‘What do you mean?’
“He said he’d hit him and he didn’t known what he’d been hit with. He said he didn’t know what happened.”
She told the jury that when she asked Toor what had happened, he replied: “I will kill you as well.”
Jurors at Wolverhampton Crown Court were sent out to deliberate on Wednesday and returned a guilty verdict after less than two hours.
Toor, 27, of Latham Crescent, Tipton, will be sentenced on March 25, and was remanded in custody until then.