Colin Nish, 42, who also played for Dunfermline Athletic and Hibs, lashed out at the woman when the pair clashed at the couple’s home earlier this year.
The attack left her with severe bruising and black eyes.
Nish had denied the attack on his now-former partner, claiming the pair were both pushing their heads together during the bust up and her injuries were caused accidentally.
But, following a trial at Edinburgh Sheriff Court on Monday, a sheriff failed to believe the retired footballer’s version of events and found him guilty of assault.
The 6ft 2ins tall ex-striker, who played for Dunfermline between 1999 and 2003 and Cowdenbeath from 2015-16, has now been banned from having any contact with the woman for the next 12 months.
The victim told the trial she and Nish had been in a relationship for about 14 years and had two children together.
She said the relationship had broken down and the pair had been arguing over money when Nish headbutted her in the bathroom of their home on April 5 this year.
She told the court: “He said to me, ‘Are you going to steal my money?’ and I said ‘yes’.
“He was in my face and then it happened. I didn’t really know what had happened as I was on the floor so quickly.”
She said Nish had then callously stepped over her and gone for a shower.
The victim said she went to a police station the following day to report the attack but, despite filling in a domestic abuse questionnaire, she did not provide a full statement.
The victim’s mother told the court that when she arrived at her daughter’s house she found her “hysterical and crying” and that the following day they had taken pictures of bruising to her face.
Nish, now of Bridge Street, Musselburgh, told the trial he was a former professional footballer who had played for clubs including Hibernian, Dundee, Kilmarnock and Hartlepool.
He said the couple had been rowing over money and the pair were both “pushing their foreheads together” at the time.
He said the clash was accidental as she had suddenly moved her head and his forehead had come into contact with her face.
However, Sheriff Ian Anderson said he believed the victim and found him guilty of butting her on the head, causing her to fall to the ground to her injury. He added: “I’m going to impose a fine on you in this case. You will be fined £500 as well as a [£20] supplement.
He also imposed a non-harassment order and told Nish “you must not do anything that might cause alarm or distress” to his former partner.