Brown, 28, arrived at Elland Road on September 9 as back-up for Shane Higgs following injury to Kaspar Schmeichel and made his debut in Saturday's 2 -1 defeat at Ipswich.
Higgs was forced off with a hamstring injury, leaving Brown as the only fit senior goalkeeper at the club. His spell has been extended until November 13.
McCartney, who arrived last month and was scheduled to return to Sunderland on October 23, has had his stay extended by two days to enable him to play in the clash against Cardiff, which has been switched to Monday October 25.
Source: PA