[5] Early-war modifications were limited to the conversion of the foremast into a tripod mast, installation of a FuMO 28[Note 1] radar with fixed antennas angled 45° to each side.

[6] T20 was ordered on 5 October 1938 from Schichau, laid down at their Elbing, East Prussia, shipyard on 28 November 1939[7] as yard number 1447,[2] launched on 12 September 1940 and commissioned on 5 June 1942; construction was delayed by shortages of skilled labor and of raw materials.

On 1–3 October, the ship conducted exercises in the Baltic with the battleship Scharnhorst, the light cruisers Leipzig and Nürnberg, the destroyers Z25, Z31 and Z37, her sisters T16, T17, T21 and the torpedo boats T22, Falke and Kondor.

On 7 March 1943, T20, T16, T21 and the torpedo boats Greif and Jaguar, joined the escorts for Scharnhorst on her voyage to the Arctic in the Skagerrak, although bad weather forced them to put into Bergen, Norway.

T20 and T21 were part of the escort force for the battleships Tirpitz and Scharnhorst and the heavy cruiser Lützow as they sailed from Narvik, Norway, to the Altafjord on 22–24 March.

[8] The ship returned to Germany in October and was assigned to the Torpedo School until March 1944 when she began a refit that lasted until August.

During 10–12 and 13–15 October, the 3rd Torpedo Boat Flotilla, with T20, T13, T16 and T21, screened Lützow and the heavy cruiser Prinz Eugen as they bombarded advancing Soviet troops near Memel.