Philip McGilvray, 33, admits biting the Game Of Thrones actor's ear but told Newcastle Crown Court he was acting in self-defence after the bigger man pinned him to the floor.
His co-accused Alan French, 32, also denies wounding with intent, and claims he was trying to stop an attack on his friend.
The pair, from Hamilton, South Lanarkshire, had been drinking and were being noisy in the Travelodge in Newcastle when Mr Mantle, 56, asked them to be quiet.
The prosecution alleged the defendants attacked him when he was on his way to complain to reception about their noise, pinned him down, and McGilvray bit his right ear.
Mantle, who was staying at the hotel while working on a touring production at the Theatre Royal in Newcastle, described the attack as like a pair of hyenas bringing down an old water buffalo.
He told the court the attack only stopped when nurse Alice Klenk, came out of her room because of the noise and grabbed both defendants by their collars "to stop them having another go at Mr Mantle".
It was not possible to re-attach the part of his ear.