Castellanos extended his hitting streak to 16 games, the longest active streak in the majors.
The banged-up Cubs had won six in a row. They were once again without first baseman Anthony Rizzo, who missed his fifth straight game due to back tightness.
Mahle (4-2) retired his first 12 batters. The right-hander allowed one hit, struck out eight and walked one.
“He’s just so tough and he’s so strong,” Reds manager David Bell said. “Tyler finds an extra gear, reserve in the tank and he’s able to get big outs.
“We’ll take that every time. Tyler continues to get better.”
Tejay Antone got four outs for his third save.
“I physically felt good,” Arrieta said. “It was just one of those games when I couldn't command the ball like I wanted to.
"It would have been nice to get the sweep. I just wasn't able to come out more crisp.”
Cincinnati jumped in front with three in the first.
After Naquin's bases-loaded sacrifice fly, Kyle Farmer hit a bouncer to Bryant at first. Bryant bobbled the ball before throwing to second for the forceout.
But the call was reversed after a Cincinnati challenge, and Tyler Stephenson was ruled safe to load the bases. Bryant was charged with an error.
Arrieta then hit Tucker Barnhart with the first pitch after the delay. India followed with an infield single to make it 3-0.
“They're huge, especially in a game where we want to avoid the sweep,” Mahle said of the first-inning runs. “It takes the wind out of their sails. We do not want to lose three straight.”
Suárez led off the fourth with a long drive to left for his 12th homer. A throwing error on third baseman Wisdom led to Naquin's two-out RBI single, chasing Arrieta.
The Cubs placed infielder David Bote on the 10-day injured list with a dislocated left shoulder. Bote got hurt sliding into second base on Saturday.
Bote had an MRI on Sunday, and there is no timeline for his return.
“I think it was a little less damaging than we thought,” manager David Ross said. “I think we got the best news we could possibly get. There were no fractures or nothing in there that was crazy off.”
Infielder Sergio Alcántara was promoted from Triple-A Iowa, and right-hander Tyson Miller was designated for assignment. The 24-year-old Alcántara pinch-hit in the sixth and popped out to second.
Reds: 1B Joey Votto (left thumb) is progressing and Bell estimated he is about a week away from returning. ... LHP Wade Miley (left foot) threw a side session Saturday and reported no ill effects on Sunday. He's expected to be activated Monday to start against Philadelphia.
Cubs: Ross said Rizzo was feeling better and hit off the tee on Saturday. Ross is hopeful he'll be able to return soon and avoid a stint on the 10-day IL. ... OFs Jake Marisnick (right hamstring) and Jason Heyward (left hamstring) each worked out before the game. Ross believes both are close to returning.
Reds: Miley (4-4, 3.50 ERA) starts the opener of a three-game home series against the Phillies. RHP Vince Velasquez (2-0, 2.95 ERA) goes for Philadelphia.
Cubs: RHP Trevor Williams (3-2, 5.36 ERA) was slated to start Monday against San Diego, but he is unavailable after he had an appendectomy Sunday morning. There was no word on who would replace him against the visiting Padres.
Soler hits 2 HRs, again, as Royals beat White Sox 4-3
— Jorge Soler hit two home runs for the second straight game and the Kansas City Royals extended their winning streak to six games with a 4-3 win over the Chicago White Sox on Monday night.
McCutchen's 3-run HR in 9th gives Phillies 6-5 win over Nats
— Andrew McCutchen hit a three-run homer in the bottom of the ninth inning to give the Philadelphia Phillies a 6-5 victory over the sinking Washington Nationals on Monday night.
Nationals hit back-to-back RBI triples vs. Phillies
Josh Bell and Josh Harrison hit RBI triples in back-to-back at-bats to open the scoring for the Nationals.
McCutchen smashes walk-off 3-run homer for Phillies
Andrew McCutchen leaves no doubt on this three-run home run in the bottom of the ninth to give the Phillies a 6-5 win over the Nationals.
Soler stays red hot with second straight multi-HR game
Jorge Soler has six homers in his last six games after crushing a pair of dingers against the White Sox.
Red Sox rally late for 2nd straight game, beat Blue Jays 5-4
— Alex Verdugo hit a two-run homer in the eighth inning and the AL East-leading Boston Red Sox rallied late for the second straight game, beating Toronto 5-4 on Monday night in the opener of an important series for the Blue Jays.