Young becomes the West Sussex side's 11th signing of the summer following his release from Bury at the end of his contract.
Crawley manager John Gregory told the club's official website: "Lewis gives us something different. He's quick and can play on either flank and even up front. We think he's got a lot to offer and it's our job to create the right environment for him to flourish.
"The signs are that we are doing that because he has been excellent in pre-season training and we saw flashes of what he is capable of on Friday against Billingshurst with his performance in the first half."
The 24-year-old has nearly 100 professional career appearances under his belt since coming through at Watford. He has since played for Hereford, Burton, Forest Green, Yeovil, Aldershot and Bury and his best stint was arguably in 2011/12 season, when he played 34 times for Northampton.
Young said: "I'm very pleased to be a Crawley player. I've been very impressed with the manager and the other lads and I am looking forward to having a successful season in League One. "It's a fresh challenge for me and I can't wait to show the Crawley fans what I can offer."