This is an irregular verb. It follows the conjugation pattern of the verb sentir.
Present:
O Miguel nunca mente para os amigos dele.
O Miguel nunca mente aos seus amigos.
Miguel never lies to his friends.
Imperfect:
Quando era criança, o Tiago mentia muito.
When he was a child, Tiago often lied.
Conditional:
A Ana nunca mentiria sobre isso.
Ana would never lie about that.