Assigned the identification number SP-2646, Avenger was commissioned on 29 May 1918 while she lay alongside Pier 19, North Wharves, Philadelphia, Pennsylvania, with Chief Boatswain's Mate Philips J. Wunderle, USNRF (her peacetime owner "called to the colors"), in command.

With low water prevailing, her sailors prepared to wait for the incoming tide to refloat Avenger, but three vessels unexpectedly arrived on the scene and offered assistance.

However, since she was shipping water due to two damaged tanks in her bottom, the yacht proceeded thence to Camden, New Jersey, for hull repairs at the boatyard of Quigley and Dorf on 11 and 12 June.

A week later, the vessel got underway at 1000, "Captain" Wunderle at the helm, and headed back toward League Island, where she took on board her main battery, a three-pounder gun, and installed it the next day.

During her second patrol, (18 to 20 July) she received information by wireless of enemy submarine activity near New York Harbor and promptly loaded her three-pounder to be ready for action.

In the pre-dawn darkness on 20 July, Avenger drew within hailing distance of Emerald and received oral orders to instruct all northbound vessels to "hug the coast" because of the U-boats operating to seaward.

The damage apparently not severe enough to force the craft to curtail her assigned tasks, Avenger returned to Cape May that afternoon, remaining alongside the Fish Dock there until 12 August when she got underway, at the end of a towline, bound for Essington, Pennsylvania.

At noon on 19 December, Chief Boatswain's Mate Wunderle decommissioned Avenger and signed the receipt for the vessel, which was later delivered to her builder's yard, Clement A. Troth's, in Camden, where she was presumably prepared for civilian service.