Assured of a place in the play-offs, DC climbed into fourth in the Eastern Conference thanks to a 2-0 win over Seattle Sounders.
Lucas Rodriguez's low strike gave DC the lead at home before Rooney helped them to their second early in the second half.
The former Manchester United star's powerful set-piece from the left was tapped in by Frederic Brillant for the host.
The Sounders are second in the Western Conference, but yet to clinch a play-off spot.
New York Red Bulls are set for the play-offs and recorded a 2-0 win over Philadelphia Union.
Tom Barlow's tap-in and Daniel Royer's late sealer wrapped up the victory for Chris Armas' men.
The fifth-placed Red Bulls are five points behind Philadelphia in the east.
Elsewhere, New York City is four points clear atop the east after a 1-1 draw with Dallas, while Portland Timbers and Minnesota United drew 0-0.