The outbreak of the American Revolutionary War in 1775, and the need to service substantial fleets at sea, strained Royal Navy resources and necessitated the purchase of additional storeships, transports and victualler vessels.

As part of this process, on 13 June 1776 the Admiralty instructed the Navy Board to obtain two new ships of around 300 tons burthen, which would resupply naval vessels then operating off the Gulf of St Lawrence in North America.

[1] The merchant vessel Union had been unarmed, so dockyard workers now cut five gunports on each side of her upper deck and filled these with four-pounder cannons.

On 21 April 1779 she set sail from New York for Portsmouth, and had reached Newfoundland Banks by 8 May, when she encountered the 20-gun American privateer General Mifflin.

The voyage was unsuccessful; on 10 May the captured vessel was sighted by the Royal Navy's 74-gun third rate, HMS Hero, and was retaken after a short pursuit.