Joiner Ryan Woolley had responded to an emergency call out and was working at a house in the darkness, last Boxing Day night when knifeman Bradley Fenwick slashed his face and almost amputated his fingers.
Newcastle Crown Court heard the 28-year-old attacker's intended target was his girlfriend's ex, who lived at the damaged house in Cedar Grove, Alnwick, and who he had warned would be put "ten feet under".
Mr Woolley, a Northumberland County Council employee, will never regain full use of his hand, has a permanent scar across his face and is unsure if he will ever work again as a joiner.
Former chef Fenwick, of Brook Court, Bedlington, was initially charged with attempted murder but later pleaded guilty to wounding with intent and having a bladed article, which was accepted.
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Judge Robert Adams sentenced him to 10-and-a-half years behind bars and told him: "He was doing the job he was employed to do.
"He was taken by surprise in that he felt a tap on the top of his shoulder. It was dark. He was immediately hit in the face by you.
"You didn't engage in conversation, no threats were made, you launched straight into a vicious attack with a sharp object, which appears to have been a meat cleaver.
"He instinctively put his right arm up to protect his face before he said he was hit another three times, four times in total. Serious injuries were caused to his face and hand."
The court heard Mr Woolley, who is in his 20s, had been working at the home of a man named Ben Brown.
Judge Adams added: "Having been struck multiple blows by you, you stopped at this point and questioned who he was. You had expected it to be Mr Brown.
"This was a premeditated attack on what you thought was Mr Brown but, in fact, you got the wrong person.
"He told you he was from the council. You looked on drink or drugs or both."
The court heard Mr Woolley needed emergency surgery after his little and middle fingers on his right hand were partially amputated. The little finger was also fractured and there was major damage to tissue and tendons.
The facial wound ran from Mr Woolley's left cheek and went through the cartilage of his nose. He had facial fractures below his eye.
He has been left with a "disfiguring" scar on his face and faces further operations.
The worker said in a statement: "I have accepted my hand won't get better but I still have to come to terms with this and try to adapt.
"I feel he has ruined my working life and my future of work is unclear."
He added: "Every day is a challenge. Because of this male my life will never be the same again."
Shaun Routledge, defending, said Fenwick who has previous convictions, had been "off the rails" as a younger man but had "pulled himself together then has committed this very serious offence".
Mr Routledge said Fenwick sent a letter of apology to the court, has the support of his family and has worked hard in custody.
He added Fenwick did not plan to attack a council employee, saying: "He was a public sector worker but he didn't know that at the time of the incident."