Mark joined the Royal Navy, possibly inspired by his father's tales of campaigning in exotic lands for the British Empire, and perhaps also by distant tales of his great-grandfather, who had battled Napoleon's forces at Waterloo. As the new century began, he found himself as a Boy, 1st Class, aboard H.M.S San Pariel after stints on the Caledonia, Minotaur and Agincourt. In 1910 he married his cousin Margaret, daughter of his uncle Donald who had served abroad with his father in the Seaforths. In 1913, with the British and German Navys trying to outbuild each other as European tensions grew, he was in the Gunnery School aboard H.M.S Excellent, before joining H.M.S Invincible - the world's first battlecrusier - at its commissioning on 3 August 1914.
"The First World War had begun. In the northern mists the Grand Fleet (21 dreadnoughts, 8 predreadnoughts, 4 battlecruisers, 21 cruisers and 42 destroyers) was at its war base in Scapa Flow, under the command of Admiral Jellicoe. Diagonally across the North Sea the German High Seas Fleet (13 dreadnoughts, 16 predreadnoughts, 4 battlecruisers, 18 cruisers and 88 destroyers) were assembling in the River Jade under the command of Admiral Von Ingenohl." - V. E. Tarrant.
Invincible was involved in three actions. It had a small part to play at Heligoland Bight later in August, and then in December was involved in a naval battle against Vice-Admiral Graf von Spee at the Falkland Islands. But the Invincible will be forever associated with the Battle of Jutland, on the last day of May in 1916, when at 6.34 p.m a salvo from the Derfflinger penetrated the 7-inch armour and causing explosions in the gun-house, turret and the magazine, rent the Invincible in two, sinking it and killing 1,019 men. There were only six survivors, and Mark Cameron was not amongst them.
To boys who had grown up with the heroic deeds of their grandfathers, fathers and uncles, or the gallant officer adventurers in the novels of G. A. Henty, who had read of the brave thin red or khaki lines defending outposts against Zulus at Rorke's Drift, or Afghans at Kam Dakka, and where casualties rarely exceeded 50 on a bad day, or 800 on a disastrous day, the Great War will have come as a shock. Over 21,000 Britons killed in the first day at the Somme in 1916, and 6,000 Britons and 2,500 Germans lost to a watery grave at Jutland is a severe lesson indeed. Today's remembrance focussed at the Cenotaph, 'that mass of national emotion frozen in stone', I always find poignant, but no lesson has actually been learned, it seems.