Philip McGilvray, 33, yesterday told his trial at Newcastle Crown Court that he acted self-defence after the Casualty and Game of Thrones actor pinned him to the floor of a hotel.
McGilvray and Alan French, 32, both of Hamilton, South Lanarkshire, deny wounding with intent during the incident at the city's Quayside Travelodge, where Mr Mantle had been staying while working locally in theatre.
Mr McGilvray said he had been drinking with his friends since the afternoon and had returned to the hotel in the early hours of March 24 last year.
He said the pair were walking along the corridor when he became aware of Mr Mantle coming out of his room to tell them to be quiet.
Mr McGilvray said: "I turned around and the next thing I was taken out by this big guy. I'm not sure if he punched me or clothes-lined me.
"It was like being hit by a car. I was on my back and Mr Mantle was straight on top of me.
"I had one arm in front of me and the other under my back and I could not move.
"He was attacking me, he was on top of me, his face right in front of me. He was pulling at me and grabbing me.
"I was surprised and shocked, I was scared, angry and I did not know what I had done.
"All I had done was speak to some girls and then I had a big monster on me.
He added: "I don't know why I did it. I could not get away, I did not know what he had in his hands. I thought he was trying to kill me."
Mr McGilvray told the court he thought he had no option but to bite Mr Mantle in a bid to stop the attack.
He said he felt ashamed for what he had done to Mr Mantle because when he was younger a bouncer bit off his own ear and swallowed it.
"I know every feeling and emotion that Mr Mantle has been through. I'm disgusted with myself and ashamed with myself," he said.
Mr McGilvray told the court he had previously been in trouble for fighting and had been convicted of assault. He also said his brother had been stabbed to death in the street.
Jolyon Perks, prosecuting, accused McGilvray of talking "absolute rubbish" and said he lied to the police when he was first interviewed.
Mr Perks said that after the altercation the pair had left the scene for three hours and eventually returned after trying to check into a different hotel.
But when the police initially interviewed him under caution, he said he could not remember what had happened and the last thing he recalled was being in a club.
"You lost your temper when you were drunk and went over the top, way over the top," Mr Perks said.
"You were unable to explain to the police in your interview what Clive Mantle could possibly have done to justify your reaction.
"You did it because when you are drunk your character changes and you decided that you were going to disfigure him."
Mr French said he was invited on the trip by McGilvray after he had split up with his partner.
Giving evidence, he too denied Mr Mantle's version of events and said he tried to stop the actor attacking McGilvray.
"I saw him lunging straight for Phil and they were straight on the floor," he said. "I turned round and saw they were wrestling on the ground."
"I started shouting, telling him to walk away, please walk away, you don't need to do this."
He told the court he was not aware at the time that McGilvray had bitten the actor's ear, an act he described as "not human".
He said: "For Mr Mantle to suggest that someone would lie down next to him and bite his ear is absurd."
He also denied punching the actor in the head.
But Mr Perks said he had failed to explain what had caused Mr Mantle to react in the way he did.
The case continues.