Magistrates in Workington heard that Mark Alan Bell, 23, had been at a christening in Whitehaven with his girlfriend Michaela Holliday on September 19.
He denied the offence but was found guilty by the court.
Bell, who was living at Miss Holliday’s house on Mossbay Road, Workington, at the time, left the party early.
In a statement, Miss Holliday told the court that Bell assaulted her by pushing and shoving her in an unprovoked attack when she returned later.
He also smeared blood from a wound he had on his hand on her sofa.
Miss Holliday ran to a neighbour, who rang the police.
Bell, who denied assault and criminal damage, claimed Miss Holliday had returned home with another man.
He told magistrates he had a fight with the man who wounded him with a flick knife.
He said: “They came in from the front door, I was angry, I said ‘this is my house’. She asked him to come back to our house.
“He went for me and I grabbed his hands. That’s when I got cut.
“I just wanted to get away. When Michaela said she was calling the police I panicked.”
Pam Fee, prosecuting, asked Bell: “Why didn’t you ring the police? There was an unknown man assaulting you in your own home with a knife and you didn’t think of calling the police. That’s because this man doesn’t exists.”
John Cooper, defending, said: “Just because someone doesn’t call the police, it doesn’t mean that something hasn’t happened. In this case, what this man is saying is his girlfriend came home with another man.”
Magistrates found Bell, guilty of the offences and released him on conditional bail until tomorrow, when he is due to be sentenced.
Miss Fee told magistrates Miss Holliday was not seeking a restraining order as the couple were back together.
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