The horizontal tube attached to a frame and command wire were left beside a derelict farm building outside the town of Castlewellan earlier this week.
A member of the public discovered the items laying unconcealed behind a wall on the outside of the structure.
Police believe the weapon, which did not contain a live mortar round, belonged to dissident republicans opposed to the peace process.
Police Service of Northern Ireland (PSNI) Detective Superintendent John McVea said it was likely that the mortar tube was left by one dissident extremist for collection by another, with the discovery disrupting its transfer.
Source: The Independent