In his final tune-up before the All-Star break, the ace right-hander was underwhelming through six innings, scattering a season-high eight hits and allowing three runs.
All three came at the hands of Rockies first baseman Tyler Colvin, who became the first player to hit two home runs in a single game against Strasburg -- a deep solo blast to right in the second and a two-run shot in the fourth.
"All I can really think of is two pitches," Strasburg said. "I don't think they were the right pitches to throw in that situation, and I didn't have the right mindset, so it should be an easy fix."
Just one other player (Atlanta's Dan Uggla) has more than one career home run against Strasburg, who had allowed only 11 through 185 innings over three seasons entering the night.
Colvin's second homer came on a 3-2 pitch, the ninth of his fourth-inning at-bat.
"He's got a very lively fastball, a changeup that drops off, and a soft curveball," said Colvin, who was facing Strasburg for the first time, and is now 10-for-17 with three homers and nine RBIs in four games against Washington this season. "He made some really, really good pitches there that I was just able to barely touch. Luckily he gave me a changeup that was a little more up."
The damage could have been worse for Strasburg, who found himself in bases-loaded trouble in the sixth with one out. But he induced a 5-2-3 double play from Rockies catcher Wil Nieves to get out of the jam still trailing just 3-0.
That would be all for the Nats' starter on the night, as Tom Gorzelanny took over in the seventh and surrendered a solo home run to Rockies center fielder Dexter Fowler. Carlos Gonzalez, 2-for-3 with a walk and two runs on the night, doubled and scored in the eighth to make it 5-0.
A Nationals offense that has so often delivered both power and timely hitting over the last two weeks was dormant until a ninth-inning rally that ultimately fell short.
After registering just two hits -- a misplayed bloop single in the fourth and an infield single in the eighth -- against Pomeranz and reliever Matt Belisle, Washington's first three hitters in the ninth reached against lefty Rex Brothers.
Ryan Zimmerman led off with a solo homer, and Michael Morse followed with a double to right. Adam LaRoche also singled, but Rafael Betancourt relieved Brothers and retired the next three hitters in order to end the ballgame.
Pomeranz earned his first win of the year by throwing 91 percent fastballs (74 of 81), breaking four bats and striking out six in 6 1/3 innings.
"I think a lot of guys are going to go home dreaming of fastballs tonight," Nationals shortstop Ian Desmond said. "It's part of the game. We just missed a bunch of balls, and they just got enough of a bunch of balls."
Strasburg now has lost three straight, and heads into the All-Star Break with an admittedly sour taste in his mouth.
"He's going to be champing at the bit to come back after the All-Star break," manager Davey Johnson said.