Zo Re-unification Organisation (ZORO) officials said Thangmawia was found unconscious inside the bathroom in his hotel room in Geneva and died after he was taken to hospital.
He was 83, and is survived by his wife, two daughters and a son.
Thangmawia, a former senator in the erstwhile underground 'Mizo government-in-exile' under the leadership of the late Laldenga, was elected as the president of the ZORO in 1990, the post he held till date.