HMS Encounter was ordered as a First-Class Sloop with screw propulsion on 5 February 1845 to be built at Pembroke, in accordance with the design developed by John Fincham, Master Shipwright at Portsmouth. Her armament was to consist of 8 guns (2 × 85 cwt, 4 × 65 cwt and 2 × 25 cwt). She was to have a more powerful steam engine rated at 360 nominal horsepower. In 1848 she would be altered abaft and lengthened at Deptford prior to completion. A second vessel (Harrier) was ordered on 26 March 1846 but after her keel was laid at Pembroke Dockyard, her construction was suspended on 9 September 1846 then cancelled five years later, on 4 April 1851. Encounter had her armament radically altered in 1850 and she was broken up at Devonport in 1866.

Encounter was the third named vessel since its introduction for a discovery vessel in service in 1616.

Harrier was the fourth named ship since the name was introduced for a ‘Cruizer’ class fir built 18-gun brig sloop, launched by Barnard of Deptford on 22 April 1804 and foundered in the Indian Ocean in March 1809.

Encounter's keel was laid in June 1845 at Pembroke Dockyard and launched on 5 September 1846. Her gun deck was 180 ft (54.86 m) with her keel length reported for tonnage calculation of 157 ft 7 in (48.03 m). Her maximum breadth was 33 ft 2 in (10.11 m) though reported 32 ft 8 in (9.96 m) for tonnage. She had a depth of hold of 20 ft 10 in (6.35 m). Her builder’s measure tonnage was 894 tons. In 1848 her gun deck was lengthened at Deptford Dockyard to 190 ft (57.91 m) with her keel length reported for tonnage calculation of 167 ft 10 in (51.16 m). Her builder’s measured tonnage increased to 935 tons with a displacement of 1,194 tons.

Her machinery was supplied by John Penn & Company of Greenwich. She shipped two rectangular fire tube boilers. Her engine was a 2-cylinder horizontal single-expansion trunk (HSET) steam engine with cylinders of 55 in (1,397.00 mm) in diameter with a 27 in (685.80 mm) stroke, rated at 360 nominal horsepower (NHP). She had a single screw propeller of 12 ft (3.66 m) in diameter.

Her armament at commissioning in 1849 consisted of a single Dundas 1853 68-pounder muzzle-loading smoothbore (MLSB) of 95 hundredweight (cwt) 10-foot naval gun on a pivot mount, one 10-inch MLSB of 86 cwt shell 9-foot 4-inch shell gun on a pivot mount and four 8-inch 65 cwt MLSB 9-foot shell guns on broadside trucks. In 1850, the initial six guns were changed to two 8-inch 65 cwt MSLB 9-foot shell guns on pivot mounts, four Dundas 1853 68-pounder 85 cwt 10-foot MLSB solid shot guns plus two 32 pounder 25 cwt 6-foot MLSB solid-shot guns on broadside trucks. Later in 1850 six more 32-pounder guns were added. In 1856 another two 32-pounders were added.

During her full power steam trials her engine developed 673 indicated horsepower (IHP) for a speed of 10.254 knots.

She was completed for sea on 12 October 1849 at a first cost of - Hull 19,734 and machinery 20,192.