Garcon can't finalize his deal with the 49ers until 4 p.m. Thursday, but when the contract is signed, league sources told ESPN's Chris Mortensen they believe he will make $16 million in its first year.
Shanahan was hired as the 49ers head coach this offseason. Shanahan was the Redskins' offensive coordinator for Garcon's first two seasons in Washington, 2012 and '13.
The 49ers released receiver Torrey Smith on Monday and were expected to scour the market for help at wide receiver. San Francisco wideouts combined for 160 receptions, 1,777 yards and eight touchdowns in 2016. By comparison, Pittsburgh's Antonio Brown had 106 catches for 1,284 yards and 12 touchdowns.
Garcon, 30, spent five years with the Redskins, fulfilling a deal he signed in the 2012 offseason, and had 376 receptions, 21 touchdowns and averaged 12.1 yards per catch. His best season with the team came in 2013, when Garcon caught 113 passes for 1,346 yards and five touchdowns while playing in then-offensive coordinator Shanahan's offense.
Garcon is known for toughness and consistency, two desired traits by coaches and quarterbacks in a receiver. The Redskins also used him better in 2016, going back to play-action passes over the middle, which opened up more run lanes after the catch. On only seven more catches this season, he gained 347 yards after the catch compared to 166 in 2015, according to ESPN Stats & Information.
For the season, Garcon caught 79 passes for 1,041 yards and three touchdowns. It was his second career 1,000-yard season -- both with the Redskins.
The Colts drafted Garcon in the sixth round out of Division III Mount Union in 2008. In four years with the Colts, Garcon caught 188 passes, averaging 13.4 yards per catch and scoring 16 touchdowns.
In six career postseason games, Garcon has caught 35 passes with three touchdowns, averaging 13.8 yards per reception.
ESPN's John Keim and Nick Wagoner contributed to this report.