"I just think he's turned a corner," manager Lloyd McClendon said after Ackley finished 2-for-4 to lift his batting average to .260. "He believes in what he's doing and he's very confident in the type of player he is, and it's showing up in the field."
Endy Chavez added an RBI double in the bottom of the eighth to provide an insurance run before Fernando Rodney picked up his 39th save in 42 chances to snap Seattle's three-game skid.
The win pushed the Mariners to 73-62 and a half-game out of the second American League Wild Card spot after the Tigers dropped a 6-2 decision to the White Sox on Sunday afternoon. It also broke an 11-game losing streak against the Nationals that dated back to 2005.
Trailing, 3-1, with one out in the fifth inning, Brad Miller and Austin Jackson got the game-winning rally started with back-to-back singles off Nationals starter Tanner Roark.
Ackley, who entered Sunday batting .314 with 18 extra-base hits since the All-Star break, then gave Seattle a 4-3 lead by hitting a three-run homer -- his 11th blast of the season -- into the right-field seats, upping his August RBI total to 24.
"I think it's just a product of seeing the ball well, getting the barrel to the ball. The home runs are going to come," Ackley said. "The better I feel, the more good pitches I swing at, the more homers I'm going to hit."
Bryce Harper staked the Nats a 1-0 advantage in the second when he launched a towering home run -- estimated distance, 418 feet -- off the window of the "Hit It Here Cafe".
"Listen, the Nats, they're a great team," McClendon said. "They're a veteran club that grind at-bats out, that make it tough on pitching."
After Nate Schierholtz added a solo blast in the top of the third to make it 2-0, the Mariners rallied in the bottom of the frame.
Making a spot start at shortstop, Miller, who finished 3-for-3, lined a one-out double before Jackson followed with an infield single. Then, Ackley cut the deficit to 2-1 with an RBI single through the left side of the infield, putting runners on first and third with one out.
Robinson Cano promptly bounced into a 6-4-3 double play.
Harper then added another home run in the fourth to give the Nationals a 3-1 cushion. It also marked Washington's 10th home run of the series.
From there, Iwakuma settled into a groove and retired the final eight hitters he faced. He allowed just five hits, fanned six and didn't walk a batter while throwing 57 of 86 pitches for strikes. Perhaps most importantly, the three homers he allowed were all solo shots.
"Well I didn't feel very good on the mound," Iwakuma said. "I didn't have a very good feel on the ball today. I kind of struggled, but I was able to change speeds, mix pitches and get out of jams."
After finishing August with a 17-10 record, the Mariners embark on a seven-game road trip, beginning Monday with a Labor Day matinee against the AL-West rival Athletics.