Julian Gressel assisted on Arriola's goal in the seventh minute and Steven Birnbaum's score in the 21st. Arriola's second goal came on 41 minutes and Nigel Robertha added some insurance in the 72nd for United (11-11-4, 37 points), which is vying for a playoff spot in the Eastern Conference and won for the third time in the last five matches.
Brandon Vazquez and Luciano Acosta recorded late goals for Cincinnati (4-13-8, 20 points), which lost a third straight contest. FCC is 0-6-2 on the road since winning three in a row from May 22-June 26.
D.C., which has won three straight and seven of eight at home, pressed from the beginning and it didn't take long for the hosts to cash in. On a brilliant cross from Gressel, via a play that started with a corner, Arriola was essentially unmarked within the box and in perfect position to convert within the first 10 minutes.
It was 14 minutes later that United went up 2-0 when Birnbaum perfectly headed home Gressel's free kick that left FCC goalkeeper Przemyslaw Tyton out of position to challenge.
Cincinnati nearly scored on consecutive attempts in the 36th minute. First, Allan Cruz's angled right-footer was saved by the sliding foot of United keeper Bill Hamid, whose rebound of that shot was directed to Acosta, who hit the left post.
United, however, didn't miss its chance right before halftime when Arriola found himself nearly alone in the box to drive home a ball that was helped by Edison Flores.
Hamid came up big again in the 63rd minute when he punched away Brenner's point-blank header. Nine minutes later, VAR confirmed Robertha's successful sprint to the net that ended with his third goal of 2021 -- assisted by Birnbaum.
D.C. is 3-0-3 all-time against FC Cincinnati, which finally dented the scoresheet through Vazquez's goal in traffic on 80 minutes. Acosta made things somewhat interesting with his goal four minutes later.