The incident took place at around 9.30 am when a masked man entered the post office armed with a knife.
A sum of money was taken and the man left the post office and made off in a red Seat Ibiza car.
The car had been stolen in a creeper burglary from a house in the Millbrook area of Larne at around 6.30am.
A member of the public who attempted to intervene in the robbery suffered cuts to his hand.
The Seat Ibiza car was later recovered in the Knocksallagh Park area of Greenisland at around 2pm and the man was arrested following a search in the Rathcoole area at around 6pm.
The man was detained on suspicion of a number of offences in relation to the robbery and recent creeper burglaries in the Larne, Carnlough and Glenarm areas.
He remains in custody helping police with their enquiries.
Detectives would ask anyone with any information or anyone discovering a knife and mask that may have been discarded following the robbery to contact CID in Larne on the non emergency number 101.