Mark Peter Bradbury of Kidgate, Louth admitted assaulting his partner Teresa Spurr at the flat they then shared at East View Terrace, Roman Bank, Skegness on May 2 during a domestic argument.
It was said that he punched her on the cheek causing her to fall to the floor when they were arguing during an evening meal.
When he was arrested he denied the assault, the court heard.
The magistrates were also told Bradbury had served a four month prison sentence for assaulting her in 2009.
Mitigating, Terry Boston said Bradbury, who he described as a ‘heavy drinker’, had left the Marines in 2005 after serving in Iraq and Bosnia but had been left ‘depressed’, for which he was seeking help.
Mr Boston said Bradbury denied punching Ms Spurr but admitted there had been a ‘significant exchange of words’ and that he had pushed her away.
The magistrates adjourned for sentence on July 5 and called for a report from the Probation Service. Bradbury was given conditional bail.