The 22-year-old will remain with Darren Sarll’s Sky Bet League Two side until the end of the season, according to the club’s official website.
King, who has previously had loan spells with Welling and Braintree, has made two appearances for Millwall this campaign, both in the Carabao Cup.
He will provide competition for Joe Fryer and Chris Day at Broadhall Way.
Cardiff remain unhappy with Derby's decision to postpone Sunday's Sky Bet Championship game at Pride Park.
Dundee United boss Csaba Laszlo has been left hoping for St Mirren to slip up in the race to seal promotion to the Ladbrokes Premiership.
Crewe will consider starting Chris Dagnall when they host Forest Green on Tuesday.