David Clarke was found guilty of stabbing Rushatharan Asok, 25, at Bab’s Food and Wine in Buckingham Street, Aylesbury.
The 63-year-old entered the shop on September 8 and asked the Sri Lankan, who had only just arrived in the UK, for assistance.
But while Asok was leaning forwards to help he stabbed him in the neck with a knife he had earlier bought from Ashco in the High Street.
The court heard how on the day of the attack on September 8 the greying pensioner walked out of the Whiteleaf Centre mental health facility in Bierton Road. After buying the knife for £5, he walked into an off-licence and bought several cans of lager, which he drank on a street corner, and then some KFC chicken.
Shocking footage shown to the court showed Clarke inside Bab’s, reaching into his jacket to pull out a kitchen knife and plunging it into the back of Mr Asok’s neck with great force.
Without speaking, he rushed out of the shop, getting a taxi back to the hospital where officers tracked him down around 45 minutes later.
Senior investigating officer, Det Ch Insp Mark Glover, of Thames Valley Police’s Major Crime unit, said: “The victim in this case was a young man who had recently come to this country to start a new life and was going about his day-to-day business at work in Aylesbury.
“Clarke had never met his victim before that day and his unprovoked actions inflicted injuries so severe upon Rushatharan that they affect his quality of life every single day.
“Rushatharan has spent a substantial amount of time receiving medical treatment and remains in hospital.
“He was left paralysed down the left side of his body as a result of this violent assault.
“I would like to thank the local community, in particular the two women who entered the shop shortly after the incident and provided first aid to Rushatharan.
“I would also like to thank all the staff and officers who have helped during this investigation, as well as the Crown Prosecution Service for bringing the case to court and securing justice for Mr Asok.”
A statement released on behalf of Mr Asok’s parents said: “We would like to thank the doctors who saved our son’s life.
“We will be forever grateful to them.
“We would also like to thank the police and everyone in the community for helping us.
“We had nothing and knew no one when we arrived from Sri Lanka, it was very scary and we were told our son would die.
“We are so grateful to everyone who prayed for him.
“Rushatharan came to England to study and to make a better life for himself.
“His life has now been changed forever and he will always need to be cared for.
“It is good to know the man who did this cannot hurt anyone else, but we get no satisfaction from the court result.
“This will affect our family for the rest of our lives.”
Clarke was remanded in custody to be sentenced at the same court on June 29.