Hart temporarily replaces Oscar Munoz who suffered a heart attack last week and is now on medical leave.
Munoz has himself only been in the top job since September when he replaced Jeff Smisek who quit amidst a corruption probe.
Hart will work closely with Henry Meyer III, non-executive chairman of the board, and United’s executive team to run the company in Munoz’s absence.
"Brett has taken on increasing responsibility beyond general counsel over the last few years in the operations and customer facing areas of the company," said Meyer.
“I am confident in his ability to continue to implement the company’s strategy and Oscar’s mission of bringing United’s people together around the shared purpose of becoming the best airline for our customers and employees."