Millwall's giant-killing feats earned Mangnall the title of, David the Giant Killer as a procession of clubs came to The Old Den and were devoured by The Lions.
Millwall's record crowd of 48,672 packed into The Den to watch Mangnall score first, with his striking partner McCartney netting the winner four minutes from time.
Millwall's dream was ended when they were paired with Sunderland in the semi-final, played at neutral Leeds Road and although Magnall scored yet again, The Lions went down 2–1.
[4] On 16 May 1939, just before the outbreak of World War II, he joined Queens Park Rangers as a player and on 29 April 1944 was offered the player-manager role.
Although initially reluctant to take it on, he did so, and he built a team that in the 1947–48 season, with George Smith as captain, won the Third Division South championship by four points to give QPR their first-ever promotion.