This would mark a reunion between Mack and 49ers HC Kyle Shanahan and fill a major need at center for San Francisco.
Rapoport adds Mack is getting $5.5 million from San Francisco.
Mack, 35, is a former first-round pick of the Browns back in 2009. After seven years with the Browns, Mack signed a five-year, $45 million contract with $28.5 million guaranteed with the Falcons in 2016.
Mack played out the final year of his deal and is slated to be an unrestricted free agent in 2021.
In 2020, Mack started 14 games for the Falcons and was rated by Pro Football Focus as the No. 20 center out of 36 qualifying players.