Sapa, 60, was declared elected by Returning Officer for the polls and state Assembly secretary Ngurthanzuala after obtaining permission from the Election Commission.
His only opponent L Ramkinlova of the Mizo National Front (MNF) secured only six votes.
A senior Congress leader and chairman of the state PSU Zoram Electronics Development Corporation, Tlau had earlier unsuccessfully contested the state Assembly polls three times.
Congress has 34 Legislators while MNF has five in the Assembly. MNF's junior partner Mizoram People's Conference (MPC) has only one Legislator.
Assembly secretariat officials said polling began at the Committee Room of the Assembly Secretariat at 9 AM and all the 40 Legislators exercised their franchise by 12 noon.