For the second straight game in which Miley has started, the D-backs committed three errors, letting their lead slip away as they dropped their second straight game, 6-5, to the Nationals at Chase Field.
"If I make good pitches, I get out of it," Miley said. "Your job is to hold the lead. I didn't do it."
The D-backs have committed nine errors in their past six games.
"To play as poorly as we did, it hurts that much more," D-backs manager Kirk Gibson said. "We're capable of much better."
Down by a run the final three innings of the game, the club had plenty of opportunities to tie the score, but couldn't break through. Aaron Hill led off the seventh with a double only to be stranded three batters later. Miguel Montero then found himself at second base with one out in the eighth and didn't move.
"We had a couple of opportunities to get the run in, but we didn't take any of them," Montero said. "It got away big time, it was a mess."
Miley ended his night allowing six runs (four earned) on nine hits over 4 1/3 innings. Five of those runs came in the fifth inning, when the first batter reached on a bunt single and two errors fueled the rest of the frame.
"You make it harder on yourself," Gibson said. "We scored five runs, but we kept giving runs away. It wasn't a very good inning, it's frustrating."
Despite not receiving help from his defense, Miley came away feeling like his command let him down for the three hits that left the infield during the fifth inning.
"I wasn't executing pitches; I was staying middle," he said. "I wasn't commanding the ball at all. I was getting away with some stuff early. A team like that, they hit fastballs. You can't throw them down the middle and expect to get away with it."
After right fielder Justin Upton's fielding error in the first inning allowed the Nationals to score first for the second consecutive game, the D-backs responded in their first at-bat when Hill homered to left field to tie the game, 1-1.
Montero gave the D-backs their first lead of the series with line drive to the left-center-field gap. Jayson Werth made a diving attempt for it and the ball rolled to the wall, giving Montero a triple after Upton led off with a double.
Three batters later, Stephen Drew compounded the damage with a two-out RBI single to put the D-backs ahead, 3-1.
Chris Johnson added a sacrifice fly in the third inning to pad the D-backs' lead before the Nationals answered in the fifth.
Washington starter and former D-backs pitcher Edwin Jackson finished his night with similar numbers to Miley, tossing 5 2/3 innings and allowing five earned runs on eight hits while striking out six.
"I thought he was throwing the ball real good and just wasn't making the pitches that he wanted to make and then he seemed to settle down a little bit," Nationals manager Davey Johnson said. "It was nice to be able to hold on when we did score some runs and he didn't have his best stuff but it was a good day for the bullpen."
Ryan Wheeler pulled the D-backs to within one with an RBI double in the sixth off Jackson, but the D-backs failed to deliver the tying run against the Nationals bullpen.
"It's not like we didn't have our opportunities," Gibson said. "We just didn't get the big hit. It didn't happen, it was disappointing."