She was 92 and is survived by six sons, family sources said.
Thanmawii was elected as a legislator from Serchhip constituency, currently held by Chief Minister Lal Thanhawla, in 1978 as a nominee of the People's Conference.
She was re-elected from Aizawl East assembly constituency in 1979 as the Brig Sailo government fell due to constitutional crisis during the last part of 1978.
Mizoram then was a union territory, before the attainment of statehood in 1987 following a historic accord signed between the Centre and the erstwhile underground Mizo National Front on June 30, 1986.