The left-back agreed a two-year deal with the O's over the weekend, bringing his time with League Two side Bristol Rovers to an end.
Sawyer made over 100 appearances for Plymouth during their time in the Championship, before spending two seasons at the Memorial Ground.
The 26-year-old told Orient's official website: "It's a great opportunity for me, a great club and I can't wait to get started. Moving back up to League One was the main reason I wanted to come here.
"You want to play football as high as you can. Last year wasn't a great one for me playing in League Two, so it really excites me to come to a club like this.
"I was born around here and moved away when I was really young. This is a great step for me to come and play my football in London."