。
She stood in front of the mirror.
Literal
She [topic-は] mirror [genitive-の] front [at-に] stood.
The relative-position phrase 鏡の前 ('the front of the mirror') uses the genitive の to mark the mirror as the reference point and 前 as the spatial relation. に here marks the location where she stood — distinct from で, which would mark a place where an action unfolds rather than the resting point. 立った is the past tense of 立つ ('to stand') — a momentary verb, hence the past for 'stood (and was now standing).'