Am I entitled to retrieve the Mahr (dowry) after discovering that my wife concealed her previous marriage and divorce from me, especially given that I asked her about it before the marriage contract and she denied any previous marital ties?

If a woman lied to you about her previous engagement or marriage contract, she is sinful. However, this lie does not justify annulling the contract, as it is not a defect that necessitates annulment. If you divorce her before consummation, she is entitled to half of the dowry. You should explain to her the prohibition of lying and the virtue of truthfulness, and proceed with the marriage if you are pleased with her religious commitment and character.