While patrolling on the first full day of the war, Amphion and her destroyers encountered and sank a German minelayer, SMS Königin Luise, but not before she had laid many of her mines.

While returning from patrolling the following morning, Amphion struck a mine on 6 August 1914 off the Thames Estuary and sank with the loss of 132 crewmen killed.

[8] By the start of the First World War in August, Amphion and her flotilla was assigned to the Harwich Force, defending the eastern approaches to the English Channel, under the command of Captain Cecil H. Fox.

In the morning of 5 August, Amphion and the 3rd Flotilla sortied into the North Sea to patrol the area between Harwich and the Dutch island of Terschelling for German activity.

At 10:15 a ship in the black, buff, and yellow colours of the Great Eastern Railway's steamers that plied between Harwich and the Hook of Holland was spotted.

On the night of 4 August, she had departed Emden and headed into the North Sea to lay mines off the Thames Estuary, which she began to do at dawn.

The destroyer Linnet attempted to tow the cruiser, but a deep crack across her upper deck showed that she was hogging badly and Fox ordered his crew to abandon ship.

One of Amphion's shells burst on the deck of the destroyer Lark, killing two of her men and the only German prisoner rescued from the cruiser.

Amphion then rapidly sank within 15 minutes of the explosion losing 1 officer and 131 ratings killed in the sinking, plus an unknown number of the crew rescued from Königin Luise.