Bayern Munich Senegalese forward Sadio Mane. AFP
Senegalese international Mane, who joined Bayern from Liverpool last month in a transfer worth a reported $42 million, 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.
Elsewhere, however, the mid-season form of MLS clubs made a difference against Premier League rivals as Minnesota United downed Everton 4-0 and Charlotte FC came out on top in a pre-season penalty shootout over Chelsea after holding the English giants to a 1-1 draw in 90 minutes.
Arsenal, meanwhile, used two second half goals to subdue Orlando City 3-1.
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.
The matches in Charlotte and Orlando both started more than an hour late because of severe weather.
In Charlotte, the excitement of seeing Raheem Sterling in Chelsea colors for the first time was tempered as the hosts came out on top 5-3 in the shootout.
Chelsea dominated from the start and saw a spate of early attempts go unrewarded before US international Christian Pulisic fired them ahead in the 30th minute with a right-footed shot over diving Charlotte goalkeeper Kristijan Kahlina.
That opener was cancelled by a Daniel Rios penalty that was converted in the final minute of injury time.
England international Sterling, signed this month to a five-year contract for a fee reported to be 50 million pounds ($59 million) after winning four Premier League titles with Manchester City, came on in the second half.
He nearly bagged his first goal in the 84th minute, but his left-footed blast was turned away.
- Arsenal pull away -
It looked as if Chelsea would still walk away with the win, but a handball by Trevoh Chalobah gave Charlotte an opening and when Conor Gallagher's spot-kick was saved in the shootout the MLS club could celebrate.
In Orlando, Arsenal notched their fourth straight pre-season win.
Eddie Nketiah scored the go-ahead goal in the 66th minute and Reiss Nelson sealed it in the 80th.
The Gunners had seized the lead in the fifth minute when Gabriel Martinelli's shot was deflected in.
Uruguayan Facundo Torres pulled Orlando level in the 29th with a blistering shot past goalkeeper Aaron Ramsdale, but with their top players brought in at halftime Arsenal pulled away after the break.
Arsenal and Chelsea will continue their preparations for the start of the Premier League season when they meet in Orlando on Saturday.
Everton head home after a disappointing two Stateside defeats. They fell 2-0 to Arsenal in Baltimore on Saturday.
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