Aizawl reported the highest number of new cases at 550, followed by Lawngtlai (20) and Champhai (16).
The new COVID-19 guidelines issued by the state government on August 8 had allowed the re-opening of schools in areas outside the Aizawl Municipal Corporation's jurisdiction.
Mizoram now has 9,510 active cases, while 39,656 people have recovered from the disease so far, including 963 on Monday.
Lockdown or other restrictions may be imposed in other places in the state by deputy commissioners depending on the pandemic situation, the release said.
The girl died at her home, where she was in isolation since July 21, he said, adding that two other patients aged 74 and 37 years died at the Zoram Medical College.
The lockdown imposed in Mizoram capital Aizawl and all district headquarters was scheduled to be lifted at 4 am on May 31.
Mizoram CM Zoramthanga, who also holds the Finance portfolio, said that the outbreak of COVID-19 has hit hard the state's economy.
Of the 139 fresh cases, 86 were diagnosed via Rapid Antigen Test, 52 through RT-PCR, and one with the help of TrueNat.
Aizawl district reported the highest 112 cases, followed by Serchhip district where 64 people were found to be positive.