The order for the Monitor M33 was given to Harland & Wolff of Belfast on 15 March. They subcontracted the building of M33 to the nearby yard of Workman, Clark & Company. The keel was laid in 1st April 1915 and the ship was launched on 22nd May. Monitor M33 was commissioned at Belfast on 17 June. She was based at Devonport and commanded by Lt. Cmdr. Q.B. Preston-Thomas. Monitor M33 was accepted from the builders and ordered to war on 24 June.
Jan 1916: Assisted the army in establishing a base at Stavros. Acted as guard ship of boom defence vessel at Salonika, the allied base for the war in Bulgaria.
May 1916: Under enemy fire assisted in salvaging guns from the damanged M30 beached on Long Island in the Gulf of Smyrna. Covered the evacuation of the wireless telegraph station and aerodrome on Long Island.
July 1916: provided covering fire during a cattle raid on the Turkish coast.
Aug 1916: Monitor M33 was detached to assist the French bombardments on the south coast of Turkey.
Dec 1916: Tasked with protecting the bridge between Euboea and mainland Greece from Greek Royalist troops.
May 1917: Bombarded enemy batteries near Sulva and Anzac beaches on the Gallipoli Peninsula.
Nov 1918: One of the vessels supervising the armistice with Bulgaria at Stavros and with Turkey at Syra.