Several firms have been associated with passenger service on the River Thames since the early 20th century. Joseph T. Mears began operations in 1907, and in 1908, the Port of London Authority was formed. That same year, the steel-hulled Viscount was constructed by Salter's of Oxford, a renowned name on the River Thames, for Mears.

Currently owned by Thompsons, Viscount is the oldest passenger boat still in service among the Dunkirk Ships. Roland Hastings has served as her skipper intermittently for thirty years; his father, Harry Hastings, participated in Dunkirk aboard the Tigris I.

Prior to the severe drought of 1976, which significantly lowered the Thames water level, Viscount regularly traveled as far as Hampton Court. However, her draft of 4 feet 6 inches posed a risk of grounding, and she now operates on the route between Westminster and Greenwich.

All Thames launches were requisitioned for Dunkirk in 1940 and sailed down the river. Some have questioned whether the steamers could have withstood exposure to sea water, but after fifty years, definitive conclusions remain elusive.

In 1956, Thames Launches converted Viscount to diesel power and undertook a significant rebuild in 1964. The wooden deck, worn and undulating from years of use, was planed down and replaced atop a steel deck. The area aft of the engine room was gutted to install a steel saloon and a new funnel.

In 1965, the wheelhouse was relocated forward to approximately 15 feet from the bow. A steel foredeck was installed, and two entrances were created to access the saloon. Passenger comfort was enhanced with a corrugated plastic awning over the foredeck and a canvas awning on the top deck to provide protection from the elements.

Although Viscount now bears little resemblance to her original configuration, these modifications were necessary to remain competitive and economically viable.

In 1977, Thompsons repositioned the wheelhouse amidships and installed a new Gardner 8LX engine. The Campion family acquired ownership in 1996. Based at Greenwich, Viscount continues to operate on the River Thames.