Originally built by Vickers for Brazil as Javary, she was purchased by the Royal Navy in 1914 on the outbreak of the First World War along with her sister ships Severn and Mersey.

The design called for monitors of 1,200 tons, that would carry two 6 inches (150 mm) guns in a single twin turret on the forward part of the ship.

This was because the shallow draught of Javary had forced her two propellers to be built into tunnels in the hull, and when going astern the shafts did not receive enough water flow to operate.

By February 1914 Javary and her sister ships Madeira and Solimoes were ready for delivery to Rio de Janeiro, but this was put on hold because the Brazilian Navy was not able to complete payment for the vessels because of the drop in the price of rubber, one of Brazil's main exports.

They sailed into the Walney Channel where Humber proved that she could no longer reach her previous speed, now running at only 9.67 knots (17.91 km/h; 11.13 mph).

This was found to be because she behaved differently when fully loaded for wartime operations, and because she had been laid up alongside for so long that her hull had been fouled with growth.

Built for operations on sheltered rivers, the vessels did not handle well sailing through the Irish Sea on their way south, and the voyage took Humber three and a half days, reaching Dover on 29 August.

[3] Continuing to serve together, the three monitors saw their first wartime service on 31 August when they were ordered to sail to Ostend to help evacuate some Royal Marines from that port.

As the Race to the Sea continued in Belgium, the three monitors were sent on 10 October to provide covering fire and protection off the coast, stationing themselves off Ostend.

[3] After their return, the three monitors joined a new squadron that included the light cruiser HMS Attentive and was under the orders of Rear-Admiral Horace Hood.

To avoid this return fire the monitors increased their distance to 10,000 yards (5.7 mi) and continued to attack targets inland.

Severn and Mersey were diverted on 28 April to East Africa, where they were to bombard the light cruiser Königsberg which was holed up in the shallow Rufiji River.

The ex-Montagu gun was then put into action instead and Humber continued, firing only sparely to conserve ammunition, to serve off the Gallipoli beaches for the next few months, helping to cover the withdrawals from Suvla and ANZAC Cove in December as the campaign came to an end.

She arrived there on 30 January 1916 and served alongside the monitor HMS M31 until the end of March, when Humber was sent to Port Tewfik in the Suez Canal to protect against any Turkish attacks there.

[10] On 4 August 1917 Humber was towed from Port Tewfik to Akaba by the armoured cruiser HMS Euryalus where she operated as a guardship.

Humber arrived in England on 10 April from where she was planned to enter a refit before travelling to the White Sea, but a change in orders meant she was sent to carry out her mission immediately, and without her sister ships.

Humber was found to be especially valuable in these operations because among the monitors she was one of the most heavily armed and protected, as well as being especially well suited for the river work because of her design for the Amazon.

She had successfully made the run to Arkhangelsk by 30 August, and in mid September she was the first monitor to return to Britain, being towed there alongside HMS M24.