The Nigerian international took both of his goals well as his side dispatched the league's bottom-dweller.
Martins grabbed a goal in the 19th minute, running onto Brad Evans' through ball down the middle and finishing low into the corner.
He capped the game with a second goal in the 91st, muscling his way through the United defense and finishing emphatically.
The result puts Seattle in sixth in the West, even on points with Los Angeles. D.C. remains in the unenviable last place in MLS.