HMS Unicorn and her sister ship, HMS Trincomalee (see entry), are surviving sailing frigates of the successful Leda class. Their lines were based on a French frigate, the Hebe, captured in 1782, and the whole class was one of the best of the age. It included such fine ships as the famous Shannon, which captured the American Chesapeake, and the Trincomalee, now under reconstruction at Hartlepool, which is the only ship afloat in Britain older than Unicorn.

His Majesty's Frigate Unicorn, of forty six guns, was built for the Royal Navy in Chatham dockyard, and she was launched in 1824. The classic sailing frigate was a fast and powerful warship, and was one of the most successful and charismatic ship designs of the age. Her length overall is 50.6 m (165.90 ft), with a displacement tonnage of 1,084, a draught of 4 m (13.11 ft) and a maximum breadth of 12.2 m (39.9 ft). Unicorn's construction, however, is far from conventional. She is a unique survivor from the brief transitional period between the traditional wooden sailing ship and the revolutionary iron steamship. 

A superstructure was built over her main deck and she was laid up "in ordinary", serving as a hulk and a depot ship for most of the next 140 years. From 1857 to 1862 she was lent to the War Department for use as a powder hulk at Woolwich, and on her return was laid up again at Sheerness. By then the sailing warship had been well and truly outclassed by steam power.

Her lack of active duty left her timbers well preserved, and in the 1960s steps were initiated to convert her to a museum ship. The initial plan was to restore Unicorn to a similar condition as her sister, HMS Trincomalee, including the addition of a totally new bowsprit. However, it was discovered that the ship was the only example of a wooden frigate of her type existing 'in ordinary', and as a result the intention is now to preserve her in her current condition.