Jason Emmerson shouldered his way into her Lincoln Street address after the woman tried to physically push him out, on March 27.
“He was aggressive and shouted: “I am going to smash your brother’s head in.”” said prosecutor Donna Fawcett.
“The defendant had clenched fists and immediate violence was anticipated.”
Once outside Emmerson kicked the wing mirror from a parked Vauxhall Vectra, leaving it dangling by wires.
He made no comment in police interview, Ms Fawcett added.
Michael Little, mitigating, said the three-year relationship had ended just after Christmas and they had a young son.
“This was a very fresh break-up,” he said. “He is very much in love with this girl.
“She has started a new relationship very quickly within the first week of the new year.”
He said that on the night Emmerson had been angered by a telephone message from the woman, asking him to drop their son off later.
Emmerson, 22, of Keswick Road, admitted using violence to secure entry to premises, criminal damage and common assault, when he appeared at Mansfield Magistrates Court, on Wednesday.
He was last in court in 2015 for dissimilar matters, magistrates heard.
Probation officer Raquia Bano said: “He became angry and in his view she had put another male before her son. He admits to being jealous.
“He accepts he should have walked away and understands the impact on his victims.”
A 12 month community order was imposed, and he was ordered to carry out 80 hours of unpaid work.
He was ordered to pay £85 court costs and a £85 government surcharge and £200 compensation.
He was banned from contacting his ex partner with a 12 month restraining order.