In January 1863, she was acquired by Leach, Harrison & Forward of Liverpool to run through the Union Navy's blockade of the Confederate coast during the American Civil War.

Britannia attempted to escape; but, at the end of a day-long chase "against a strong wind and sea" in which the Union sidewheel steamer slowly gained on the fleeing ship, Santiago de Cuba, about 7:00 p.m., finally was close enough to open fire.

Commander Robert H. Wyman, the captain of Santiago de Cuba, placed a prize crew under Acting Master Edgar C. Merriman on board Britannia and sent her to Boston, Massachusetts.

On the morning of 8 April, Britannia steamed within range of Confederate coastal batteries to take under tow a small sailing ship which seemed to be drifting helplessly toward the Southern shore.

On the evening of 6 May, Flag Officer William Francis Lynch, CSN—in the Southern ironclad steam sloop Raleigh—led a small naval force out of the Cape Fear River and over the bar at New Inlet.

After crossing that barrier, Lynch headed straight for Britannia which promptly sent up several rockets to warn her sister blockaders of the impending attack and fired her 30-pounder Parrott rifle at Raleigh.

Eager for action, the determined Southerner continued her advance toward the fleeing blockader; but her consorts, the steam gunboat Equator and the tug Yadkin turned back.

John Taylor Wood, CSN, the swashbuckling grandson of President of the United States, Zachary Taylor—as that Confederate cruiser was returning from one of the South's most successful commerce raiding cruises and was attempting to slip through the blockade to safety in Wilmington.

Tallahassee, joined by Southern batteries emplaced on the Mound, fired back at Britannia; and shrapnel from one shell which burst close aboard caused some damage to the Union blockader.

Soon thereafter, Britannia began preparations to take part in a joint Army-Navy expedition against Fort Fisher which Confederate forces had erected to control the New Inlet entrance to the Cape Fear River.

Transports carrying the Army troops had retired to Beaufort in order to avoid the anticipated effects of the explosion, and fleet units had assembled in a rendezvous area 12 miles from the fort.

The staunch Southern defenders, under the command of the Col. William Lamb, were driven from their guns and into the bombproofs of Fort Fisher, but managed to return the Federal fire from a few of their heavy cannon.

The Army transports returned to Hampton Roads to prepare for a second move on the Confederate bastion, while Porter's fleet, including Britannia, remained in the Wilmington-Beaufort area and continued sporadic bombardment in an effort to prevent repair of the fort.

Then, once the assault teams were on the sandy beaches, she intermittently shelled the Confederate works during the bloody three-day struggle which culminated with the surrender of the gallant, but beleaguered, Southern garrison on the evening of the 15th.

She reached Key West, Florida, in mid-February and, about a week later, left that port with a joint Army-Navy force which had been assembled to attack Fort Ward, up the St Marks River.

Britannia managed by great effort to get higher than most of her consorts, but a report that the Army was retreating prompted the naval force to begin to withdraw on 7 March.