What transportation is available from airport to the Mong Kok after midnight and until what time?

Yeah, even at midnight, you can take the night bus N21 from the airport to Bank Center Nanthan Road. The bus alighting site is just beside the Mong Kok MTR Station. The bus fare is HKD23, and the travel time is about 75 mins.

The buses are available from 00:20 to 04:40, running at an interval of 20 minutes around.