Intended for service in Asia, Canopus and her sister ships were smaller and faster than the preceding Majestic-class battleships, but retained the same battery of four 12-inch (305 mm) guns.

At the beginning of the First World War in August 1914, she was mobilised for service in the South America Station, where she patrolled for German commerce raiders.

Moored at Port Stanley as a defensive battery, she fired the first shots of the Battle of the Falklands in December, which led Spee to break off the attack before being chased down and destroyed by Admiral Doveton Sturdee's battlecruisers.

She participated in major attacks on the Ottoman coastal fortifications defending the Dardanelles in March 1915, but the British and French fleets proved incapable of forcing the straits.

As was customary for battleships of the period, she was also equipped with four 18-inch (457 mm) torpedo tubes submerged in the hull, two on each broadside near the forward and aft barbette.

[6] Captain Philip Francis Tillard was appointed in command on 1 December 1902,[7] and in January 1903 she carried the British Secretary of State for War, St John Brodrick, during a visit to Malta and Gibraltar.

Returning to the commissioned Reserve at Portsmouth, she was rammed by the battleship HMS Barfleur in Mount's Bay during manoeuvres on 5 August 1904, suffering slight damage.

On 27 October, Cradock detached his light cruiser Glasgow to Coronel to gather intelligence, and the next day ordered Canopus to bring the colliers to the Juan Fernández Islands, where his squadron would replenish its fuel.

Glasgow arrived in Coronel on 31 October, but departed too early on 1 November to receive an order from the First Sea Lord that Cradock should not risk engaging Spee's squadron without Canopus.

"[16] Shortly after news of the battle reached Britain, the Royal Navy ordered all naval forces in the region to consolidate; this included the remnants of Cradock's command, along with the armoured cruisers Defence, Carnarvon, and Cornwall.

[17] Canopus and Glasgow returned to Stanley, arriving there on 8 November 1914; they immediately proceeded to join the British warships concentrating off the River Plate.

Canopus herself was beached in the mudflats[10] in a position that allowed her to cover the entrance to the harbour and have a field of fire landward to the southeast; to reduce her visibility, her topmasts were struck and she was camouflaged.

[19] On 7 December, the main British squadron, commanded by Vice Admiral Doveton Sturdee, arrived in Stanley and began coaling, with the intention of departing two days later to search for Spee.

Under fire from Canopus and spotting the tripod masts of Sturdee's battlecruisers, Spee called off his force's planned attack on the Falklands.

Sturdee's battlecruisers, much faster than Spee's ships, eventually caught and destroyed the East Asia Squadron, with the exception of the light cruiser SMS Dresden, which was able to outrun the British pursuers.

Canopus and Swiftsure were tasked with suppressing the guns in the fortress at Dardanus while Cornwallis would engage minor batteries at Intepe and Erenköy.

[24] The heavy Ottoman fire forced Canopus and Swiftsure to withdraw from their bombardment position, though this placed them in range of the guns at Erenköy as well, while those at Dardanus could still engage them.

The three ships then withdrew, seemingly having achieved their objective, though that night, when destroyers and minesweepers tried to clear the minefields blocking the straits, they were met with very heavy fire and were forced to withdraw.

[9] During the 18 March attack, a fleet of British and French warships—including Queen Elizabeth and the battlecruiser Inflexible—would attempt to suppress the forts in daylight, allowing the minesweepers to finally clear the fields unmolested by Ottoman fire.

[29] After that attack, Canopus and protected cruiser HMS Talbot escorted the damaged Inflexible from Mudros to Malta on 6 April.

In heavy weather and with Inflexible nearly foundering, Canopus had to take the crippled battlecruiser under tow, stern first, during the last six hours of the voyage on 10 April.

[30] Returning to the Dardanelles, Canopus took part in the blockade of Smyrna[10] and covered a diversionary attack on Bulair during the main landings on 25 April 1915.

[31] When her sister ship Albion became stranded on a sandbank off Gaba Tepe under heavy fire on 22–23 May 1915, Canopus towed her free.

[32] On 25 May, Canopus withdrew to Mudros and while leaving the Dardanelles, encountered the German U-boat SM U-21, which went on to sink the battleship Triumph later that day.

[9] After the Dardanelles campaign ended with the evacuation of Allied forces from Gallipoli in January 1916, Canopus was assigned to the British Eastern Mediterranean Squadron, where she served until she returned to the United Kingdom in April 1916.