"Ivanhoe" was in transit between Alexandria and Malta when World War II began in September 1939, but she was in Plymouth on 14 September as the entire 3rd Destroyer Flotilla had been transferred to the Western Approaches Command for escort duties. Together with her sisters, , , and , the ship sank the on 14 October. She was refitted at Sheerness Dockyard and converted to a minelayer from 14 November–13 December. "Ivanhoe" was transferred to the specialist minelaying 20th Destroyer Flotilla on 12 December and laid her first minefield, along with the other three ships of the flotilla, at the mouth of the Ems estuary on the night of 17/18 December. Another minefield was laid on the night of 2/3 January 1940 by "Ivanhoe" and "Intrepid" and they then laid a series of anti-submarine minefields later in the month. The ship replaced her guns and torpedo tubes at Portland from 27 January–3 February and resumed her former duties.