。
It was years later that mother finally told me the truth.
Literal
Mother [subject-が] me [to-に] truth [obj-を] told-for-me [topic-のは] many years after [becoming-になって] was.
A cleft construction: [clause]のは[something]である/だ highlights the 'something' as the focus — here 'when' mother told the truth. 何年も後になって literally 'becoming many years later,' with になる marking the temporal result. であった is the formal/literary past of である. The whole thing has a written, reflective flavor suitable for memoir-style narration.