With the Mariners holding a 1-0 lead, Michael Saunders began the fifth with a walk off Rangers starter Scott Feldman (0-2) and Mike Carp followed with a single. One out later, Dustin Ackley was intentionally walked to get to Liddi, who smacked a pitch off the facing of the second deck in left field for a grand slam.
The home run broke the contest open and handed Seattle the rubber match of a three-game set.
Millwood (3-4) won his third straight start -- he has allowed only one run through 22 innings over the span. The veteran right-hander surrendered three hits, walked one and fanned two over six frames Wednesday.
Adrian Beltre hit a two-run homer for the American League West-leading Rangers.