Even if she felt unpleasant, she never let it show on her face.
Literal
She [topic-は] unpleasant thought [even if-としても], never face [on-に] didn't-show.
決して顔に出さなかった — 決して ('never') pairs with a negative to express absolute never. 顔に出す ('to show on one's face') is a common expression for revealing emotions.