Chawngthu would be the only woman legislator in the 40-member state Assembly and the first single woman legislator in the history of the state.
She also happened to be the first woman legislator in the state after a gap of 27 years.
Daughter of a senior Congress leader and former minister C Chawngkunga, she is president of the state PCC youth.
The swearing-in ceremony was attended by Mizoram Chief Minister Lal Thanhawla, his Cabinet colleagues, Deputy Speaker, parliamentary secretaries and legislators