United got their first win of the new season at Fratton Park.
Darren Fletcher scored his second goal in two games, building the ball home from Patrice Evra's cross after 32 minutes.
Pompey's best efforts came after the break but Jerome Thomas and Papa Bouba Diop saw their shots saved comfortably.
Fortunately, United has restoring striker Carlos Tevez and Anderson to their line-up, Pompey were always likely to be streched. Man Utd dictate the play and pull off Portsmouth out of position.
The result leaves Portsmouth rooted to the foot of the table, while United move up to third.
Manchester United manager Sir Alex Ferguson said:
"We played some excellent football tonight. This is difficult place to come, so it is a bonus to come here and win."
"We tried to play a system where the players would be comfortable. We wanted to have Anderson playing centrally, close to Carlos Tevez."
"We let Paul Scholes control the game, and used Darren Fletcher's energy."
Van der Sar, Brown, Ferdinand, Vidic, O'Shea, Fletcher, Anderson (Possebon 76), Scholes, Evra, Rooney, Tevez.
Subs not used: Kuszczak, Neville, Fabio Da Silva, Rafael Da Silva, Gibson, Campbell.
Booked: Brown, Fletcher.
Goal: Fletcher 32.
Referee: Chris Foy
Man of the Match: Darren Fletcher.
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