Horton joined the Royal Navy officer training ship, HMS Britannia on 15 September 1898.
Whilst on HMS Duke of Edinburgh, he was involved in the rescue efforts when SS Delhi ran aground off Cape Spartel and was subsequently awarded the Board of Trade Medal for Saving Life at Sea in silver.

The outbreak of war saw Lieutenant-Commander Horton in command of one of the first British ocean-going submarines, the 800-ton HMS E9.
At dawn on 13 September 1914, he torpedoed the German light cruiser SMS Hela six miles southwest of Heligoland.
Hela was hit amidships with the two torpedoes, fired from a range of 600 yards.
All but two of her crew were rescued by the U-18 and another German ship.
Although pursued most of the day by German naval forces, E9 managed to reach Harwich safely.
Entering the port, Horton initiated the tradition of British submariners of hoisting the Jolly Roger after a successful patrol.

Three weeks later, Horton sank the German destroyer S116 off the mouth of the river Ems.
For sinking the cruiser and the destroyer, Horton was awarded the Distinguished Service Order (DSO).
Sent to the Baltic Sea as part of a British flotilla, Horton sank a number of merchant vessels and, on 2 July 1915, damaged the German armoured cruiser SMS Prinz Adalbert.
On 31 December 1914, Horton was promoted to Commander.

In 1917, Horton was awarded the bar to his DSO for long and arduous services in command of overseas submarines.
Three years later, as a captain, he was awarded a second bar to his DSO for distinguished service in command of the Baltic submarine flotilla.
Horton joined the Royal Navy officer training ship, HMS Britannia on 15 September 1898. Whilst on HMS Duke of Edinburgh, he was involved in the rescue efforts when SS Delhi ran aground off Cape Spartel and was subsequently awarded the Board of Trade Medal for Saving Life at Sea in silver.
The outbreak of war saw Lieutenant-Commander Horton in command of one of the first British ocean-going submarines, the 800-ton HMS E9. At dawn on 13 September 1914, he torpedoed the German light cruiser SMS Hela six miles southwest of Heligoland. Hela was hit amidships with the two torpedoes, fired from a range of 600 yards. All but two of her crew were rescued by the U-18 and another German ship. Although pursued most of the day by German naval forces, E9 managed to reach Harwich safely. Entering the port, Horton initiated the tradition of British submariners of hoisting the Jolly Roger after a successful patrol.
Three weeks later, Horton sank the German destroyer S116 off the mouth of the river Ems. For sinking the cruiser and the destroyer, Horton was awarded the Distinguished Service Order (DSO). Sent to the Baltic Sea as part of a British flotilla, Horton sank a number of merchant vessels and, on 2 July 1915, damaged the German armoured cruiser SMS Prinz Adalbert. On 31 December 1914, Horton was promoted to Commander.
In 1917, Horton was awarded the bar to his DSO for long and arduous services in command of overseas submarines. Three years later, as a captain, he was awarded a second bar to his DSO for distinguished service in command of the Baltic submarine flotilla.
Max Horton