Dustin Ackley hit a two-run single in the fourth to back Millwood (5-12), who allowed four hits and one run in his first victory since July 28. The right-hander had gone 1-8 since May 21.
Millwood walked three and struck out three. Tom Wilhelmsen worked the ninth for his 24th save in 27 opportunities as the last-place Mariners (67-71) won for the 11th time in 14 home games and matched their win total from last season with 24 games remaining.
Eric Thames opened the fourth with a bloop single to left. Mike Carp walked and Casper Wells singled through the left side, loading the bases with nobody out.
After Brendan Ryan popped out, Ackley ripped Aaron Cook's fastball up the middle.
Cook (3-9) also went six innings, giving up seven hits. He walked three and struck out five.
The last time Cook faced the Mariners on June 29 he threw 88 pitches in a two-hit shutout for his first complete game in more than three years. Since then, he is 1-8. His last victory was Aug. 6.
The Red Sox threatened in the fourth with two outs. Cody Ross, who had three hits, and James Loney reached on singles and Jarrod Saltalamacchia walked. Ryan Lavarnway bounced out to third to end the inning.
Boston finally pushed a run across in the sixth. Jacoby Ellsbury reached on a fielder's choice, stole second and scored on Loney's double into the right-field corner.
Seattle's opponents have scored one run or less in 33 games this season, trailing only the New York Mets (34) for the major league lead. The franchise record is 37 set in 2001.
NOTES: Red Sox manager Bobby Valentine said when Triple-A Pawtucket finishes the season the club could add "maybe one or two" more players. "I know (GM) Ben (Cherington) is looking around as to who we might add to the roster." Valentine would like to see at least another outfielder. ... OF Ryan Kalish intends to resolve his lingering shoulder issues during the offseason. "He's really concerned with something being done properly," Valentine said. "I don't think it's surgery." ... Dustin Pedroia was hitless in three at-bats, ending his season-high 15-game hitting streak. ... Mariners OF Michael Saunders (hamstring), who has missed 10 of the last 11 games, and OF Franklin Gutierrez (groin) should both be available to play Friday, according to manager Eric Wedge. ... The Mariners have only three games remaining against a team with a losing record (Toronto).