The thief, wearing a balaclava and wielding a baseball bat, was forced to flee empty-handed.
A car was seen being driven erratically in Carrickfergus before a man got out of it carrying a gun.
The attack was an intimidation attempt by a paramilitary group, says the garage owner.
Peter Johnston, who won in an election with a 22% turnout, is the son-in-law of DUP MP Paul Girvan.
The victim was hit with a fire extinguisher and suffered a fractured skull in Carrickfergus attack.