Trumbo walked with one out then advanced to second on a groundout. Hardy then muscled a 1-2 pitch from right-hander Blake Wood (0-2) into left field for the winning run.
Darren O'Day (1-1) pitched one inning with two strikeouts to earn the victory. Brad Brach retired the Reds in order in the 10th for his second straight save.
Jonathan Schoop homered for Baltimore (10-4), which took two of three from Cincinnati. Joey Votto went deep for the Reds (9-7).
Orioles starter Wade Miley allowed only one hit through seven innings as Votto's solo home run in the fourth tied the score 1-1.
After retiring nine straight into the eighth, Zack Cozart doubled with one out. Miley struck out the next two batters to strand the go-ahead run at second.
Miley tied a career high with 11 strikeouts, reaching that total for the third time in his career. Opponents are hitting .127 against him through three starts this season.
Both starting pitchers rolled through the first six innings, allowing a combined two runs and five hits.
Baltimore, which struck out 16 times Wednesday night but won 2-0, was limited to a run and four hits by right-hander Scott Feldman on Thursday. Feldman struck out four and did not walk a batter through seven innings over 86 pitches.
Reds right-hander Raisel Iglesias struck out five over two innings of relief.
Feldman retired the first five batters he faced before Schoop crushed his first pitch into the left-field seats to put the Orioles ahead 1-0. It was his third homer this season.
Miley rolled through the first three innings, retiring nine straight on only 36 pitches. He set down the first 11 he faced before Votto launched a 2-0 pitch into the visitor's bullpen in right center to tie the score at 1-1. Votto's fifth home run of the season was estimated at 423 feet.
Baltimore had runners thrown out at third base in consecutive innings.
Machado was thrown out by right fielder Scott Schebler in the fourth inning trying to reach third on Trumbo's single. Then, in the fifth, Hardy doubled over Billy Hamilton's head in center but was retired on a relay from shortstop Cozart trying for a triple.
NOTES: Orioles manager Buck Showalter announced that LHP Jayson Aquino will make his major league debut Saturday against the Boston Red Sox. ... Reds C Devin Mesoraco will remain with Double-A Pensacola through the weekend. Manager Bryan Price said Mesoraco could be activated on April 28 following an off-day. ... Orioles LF Seth Smith missed his second straight game with a strained right hamstring.