HMS Ark Royal was the first ship designed and built as a seaplane carrier. She was purchased by the Royal Navy in 1914 shortly after her keel had been laid and the ship was only in frames; this allowed the ship's design to be modified almost totally to accommodate seaplanes. During the First World War, Ark Royal participated in the Gallipoli Campaign in early 1915, with her aircraft conducting aerial reconnaissance and observation missions. Her aircraft later supported British troops on the Macedonian Front in 1916, before she returned to the Dardanelles to act as a depot ship for all the seaplanes operating in the area. In January 1918, several of her aircraft unsuccessfully attacked the German battlecruiser SMS Goeben when she sortied from the Dardanelles to attack Allied ships in the area. The ship left the area later in the year to support seaplanes conducting anti-submarine patrols over the southern Aegean Sea.

After the end of the war, Ark Royal mostly served as an aircraft transport and depot ship for those aircraft in support of White Russian and British operations against the Bolsheviks in the Caspian and Black Sea regions during the Allied intervention in the Russian Civil War. She also supported Royal Air Force (RAF) aircraft in British Somaliland in the campaign against Mohammed Abdullah Hassan in 1920. Later that year, the ship was placed in reserve. Ark Royal was recommissioned to ferry an RAF squadron to the Dardanelles during the Chanak Crisis in 1922. She was reduced to reserve again upon her return to the United Kingdom the following year.

The ship was recommissioned in 1930 to serve as a training ship, for seaplane pilots and to evaluate aircraft catapult operations and techniques. She was renamed HMS Pegasus in 1934, freeing the name for the aircraft carrier ordered that year, and continued to serve as a training ship until the beginning of the Second World War in September 1939. Assigned to the Home Fleet at the beginning of the war, she took on tasks as an aircraft transport, in addition to her training duties, until she was modified to serve as the prototype fighter catapult ship in late 1940. This type of ship was intended to defend convoys against attacks by German long-range maritime patrol bombers by launching fighters via their catapult to provide air cover for the convoy. Pegasus served in this role until mid-1941 when she reverted to her previous duties as a training ship. This lasted until early 1944 when she became a barracks ship. The ship was sold in late 1946 and her conversion into a merchant ship began the following year. However, the owner ran out of money during the process and Anita I, as she had been renamed, was seized by her creditors in 1949 and sold for scrap. She was not broken up until late 1950.

The Royal Navy had conducted trials in 1913 with a modified cruiser, Hermes, to evaluate the ability of seaplanes to work with the fleet. They were successful enough that the Admiralty allocated £81,000 in the 1914–1915 Naval Programme to purchase a merchant ship for a more thorough modification than had been possible with Hermes to better accommodate seaplanes. A tramp steamer was purchased in 1914 that had just begun construction at the Blyth Shipbuilding Company so it could be easily modified to suit its new role.

Ark Royal was laid down on 7 November 1913 by the Blyth Shipbuilding Company in Blyth, Northumberland, as a freighter, probably intended for the coal-for-grain trade in the Black Sea. She was purchased in May 1914 and was launched on 5 September 1914. The ship was commissioned on 10 December 1914.

Extensive changes to the ship were made in converting her to a seaplane tender, with the superstructure, funnel, and propulsion machinery moved aft and a working deck occupying the forward half of the ship. The deck was not intended as a flying-off deck, but for starting and running up of seaplane engines and for recovering damaged aircraft from the sea. The ship was equipped with a large aircraft hold, 150 feet (45.7 m) long, 45 feet (13.7 m) wide and 15 feet (4.6 m) high along with extensive workshops. Two 3-long-ton (3.0 t) steam cranes on the sides of the forecastle lifted the aircraft through the sliding hatch of the hangar onto the flight deck or into the water. She carried 4,000 imperial gallons (18,000 L; 4,800 US gal) of petrol for her aircraft in standard commercial 2-imperial-gallon (9.1 L; 2.4 US gal) tins.

She could carry five floatplanes and two to four wheeled aircraft. The seaplanes would take off and land in the water alongside the carrier, lifted on and off the ship by cranes; the other aircraft would have to return to land after launch. Her original complement of aircraft consisted of a Short Folder, two Wight Pushers, three Sopwith Type 807 seaplanes and two to four Sopwith Tabloid wheeled aircraft.

Ark Royal had an overall length of 366 feet (111.6 m), a beam of 50 feet 10 inches (15.5 m), and a draught of 18 feet 9 inches (5.7 m). She normally displaced 7,080 long tons (7,190 t), with a displacement of 7,450 long tons (7,570 t) at deep load. The ship had one vertical triple-expansion steam engine driving one propeller shaft. The ship's three cylindrical boilers generated enough steam to produce 3,000 indicated horsepower (2,200 kW) from the engine. The ship had a designed speed of 11 knots (20 km/h; 13 mph); she made a speed of 10.64 knots (19.71 km/h; 12.24 mph) during her sea trials with 2,675 shaft horsepower (1,995 kW) in December 1914. Ark Royal carried 500 tonnes (490 long tons) of fuel oil, enough to give her a range of 3,030 nautical miles (5,610 km; 3,490 mi) at 10 knots (19 km/h; 12 mph).