The 25-year-old has joined the Sky Bet League One club on a two-year contract and is looking forward to helping them push for promotion after the disappointment of last season.
Lowry has made the move to Brisbane Road despite being a regular in the Lions team over the past two years.
The Australian made 23 appearances last season and was a regular in 2012/13, but has his mind set firmly on achieving promotion at his new club.
He told Orient's official website: "I'm really excited to be here.
"There's a buzz about the place. You can tell straightaway that everyone here is really ambitious and I want to be a part of it and help the club push to go up this year."
Lowry is Orient's fourth summer signing as they aim to bounce back from the disappointment of losing in the play-off final on penalties to Rotherham.