A device for mounting of a plumbing fixture such as a faucet or the like through a hole in a counter, a sink or the like and connection of the fixture to a supply line. A collar member of the device is slidingly retained on an inlet tube of the fixture by a connecting nut of the supply line. A rod, guided in its axial movement by a guide member that is connected to the collar member, has a flag member that is rotatable between a stowed position and a deployed position. In the stowed position, the device may be passed through the hole when connected to the plumbing fixture and the supply line. In the deployed position, the flag member increases the axial profile of the device and co-acts with the body of the plumbing fixture in retaining the plumbing fixture in the hole.