The Chicago Bears made Mills a fifth-round selection out of Louisiana Tech in 2013. Mills, a six-year veteran, joined the Buffalo Bills midway through the 2015 season and started all 16 games for the team in each of the past three seasons.
Mills spent his first two seasons with Chicago, a very brief stint with the Dallas Cowboys in 2015, and then four seasons in Buffalo. He signed a one-year, $3 million deal with the Miami Dolphins on May 9 but was released on August 27.
San Francisco currently has Daniel Brunskill and rookie Justin Skule backing up starting tackles Joe Staley and Mike McGlinchey.