"It felt pretty good," the 23-year-old rookie said. "It was a good moment. Obviously, you see the big-time guys doing it and when you do it, it's a good thing."
Millwood, 37, gave up just three hits with one walk and had two strikeouts to move past Catfish Hunter into 64th on MLB's all-time strikeout list with 2,013.
Texas scored three runs in the eighth off reliever Tom Wilhelmsen before Brandon League came on in the ninth for his ninth save.
Millwood has been outstanding his last three starts, including a two-hit shutout at Colorado in his previous outing on Friday. But this was easily his best start at Safeco Field, where he was 0-2 with a 10.05 ERA in his first three games as opposed to 2-2 with a 1.78 ERA in five road appearances.
Millwood has thrown 17 straight scoreless innings and is 3-4 with a 3.72 ERA after starting the season 0-4 with a 5.88 ERA in his first six starts.
"I think the biggest thing is just confidence has been building the last couple weeks, so when I got out there, it just felt like I was going to have a good game and throw the ball well," said Millwood, who still knows most of the Rangers from his time with Texas from 2006-09. "When I missed, guys made good plays or they just fouled it off. Confidence goes a long way in this game and that's kind of what I'm working on right now."
The Mariners won the series with Texas, improving to 21-25 on the season heading into a four-game Safeco set with the Angels. Seattle is 3-4 vs. the Rangers this season after finishing 4-15 against them a year ago.
Texas (27-18) totaled just seven runs and 16 hits in the three games against the Mariners, whose starters are 5-2 with a 1.42 ERA over the past seven outings. Millwood is 3-0 with a 0.41 ERA in his own last three starts and the Rangers came away impressed.
"For some reason, my timing always seems off against him," left fielder David Murphy said. "He's not much to figure out -- he's always been fastball, curve, slider and changeup. But he's a veteran who knows how to pitch and has been doing it for a long time.
"He's had our number and he's in our division, so we're going to face him a lot and we have to make an adjustment. He also caught us at a time when our offense is not producing the way we're capable of. But you have to give him credit."
Seattle managed just four hits for the game, three coming against starter Scott Feldman, but they took advantage of five walks in his 4 1/3 innings to score five times.
Liddi's slam -- the second of the season for Seattle -- certainly helped. The big Italian cleared the bases after Michael Saunders walked, Mike Carp flared a single just over shortstop Elvis Andrus and Dustin Ackley was intentionally walked after an error on a pickoff attempt at second base had pushed both runners into scoring position.
"That was huge," Millwood siad. "One run against those guys, you never ever feel comfortable. For him to put us up 5-0 let me relax a little and let our offense and defense relax and we could all just go out and play the game."
Liddi missed a fly ball in the corner by Yorvit Torrealba for a two-base error leading off the third. But Millwood stranded Torrealba at third to escape any harm, and the youngster more than made up for that with his bat.
Liddi doubled in the first inning and went 2-for-3 with a walk. He's hitting .273 with three home runs and nine RBIs in 77 at-bats.
"He's still a young hitter, learning how to hit," manager Eric Wedge said. "But the ball comes off his bat as good or better than anybody we have."
Feldman, starting in place of Mariners nemesis Neftali Feliz, walked the first three batters in the second inning and Saunders drove one run in on a sacrifice fly to center, but that was the only damage the Mariners could do with that opportunity as they finished the day 1-for-8 with runners in scoring position.
The one, of course, was Liddi's slam. And that was enough to help Seattle snap a string of five straight series losses to the two-time defending American League champions.