Hernandez (3-1) yielded only a fourth-inning single to Denard Span in the dominating performance, retiring the 13 final batters he faced.
The Mariners ace struck out nine and walked a pair while placing 71 of his 107 pitches for strikes and lowering his ERA to 1.89.
Steve Delabar pitched a 1-2-3 ninth to complete the one-hit shutout as Seattle evened its three-game set with Minnesota.
Seager's two-run single in the sixth inning opened the scoring and he added a two-run homer to cap a five-run seventh for the Mariners.
Ichiro Suzuki delivered a two-run triple in the big inning.
Jason Marquis (2-1) absorbed the loss for the Twins, who on Thursday were no-hit by the Los Angeles Angels' Jered Weaver. The Minnesota right-hander struck out six over six innings, allowing two runs on four hits.