The Fire (12-12-7) moved past Philadelphia, Columbus, and New England into the fifth and final playoff spot in the Eastern Conference.
United (3-22-6) are now winless in their last nine league games and were unable to carry over any momentum after winning the US Open Cup for the third time in franchise history during the week.
Larentowicz opened the scoring in the 24th minute with his second goal of the season as Juan Luis Anangono took a United turnover and dribbled past two defenders before finding the Fire captain inside the penalty box for the close-range score.
Two minutes later, Soumare doubled the Fire lead with his first goal of the season. The defender capitalized after United goalie Joe Willis and defender Perry Kitchen were both unable to clear away Mike Magee's free kick from the right wing.
Amarikwa, who entered the game in the 79th minute, scored 10 minutes later after fellow substitute Dilly Duka found him all alone after a short corner kick in the closing minutes.
D.C. United dominated possession in the first half but were unable to beat Fire goalie Sean Johnson from any angle.
United's best chance may have come in the 15th minute when Jared Jeffery ripped a shot from 25 yards out that finally beat Johnson only to come off the left goal post. United's Connor Doyle immediately put the rebound right back on frame only to have Johnson again make the save.
Johnson made five first half saves -- finishing with nine -- on the way to his fifth shutout of the season.