Senegalese international Mane, who joined Bayern last month in a transfer worth a reported $42 million after a six-year stint with English giants Liverpool, opened his account for the Bundesliga champions from the penalty spot after five minutes.
Mane’s spot-kick was awarded after D.C. United centre-back Donovan Pines upended Lucas Copado as the teenage striker surged into the penalty area.
The early strike set the tone for a one-sided clash at Washington’s Audi Field that underscored the work facing former England and Manchester United star Wayne Rooney, recently installed as manager of the struggling MLS side.
Bayern doubled their lead in the 12th minute when Marcel Sabitzer’s shot from outside the area wrong-footed D.C. United goalkeeper Jon Kempin.
Serge Gnabry made it 3-0 just before half-time, tucking away a low cross from Mane in the 44th minute.
De Ligt then marked his move from Italy’s Juventus earlier this week with a goal on debut after coming on as a substitute at half-time.
The Dutch defender crashed in a volley from a corner to make it 4-0 after 47 minutes.
Joshua Zirkzee added a tap-in after a patient build up from Bayern to make it 5-0 in the 51st minute before Skage Lehland’s consolation effort for D.C. United three minutes later.
Theodore Ku-Dipietro added a second for D.C. United three minutes from time before German veteran Thomas Mueller made it 6-2 deep into injury time.