The 21-year-old has been training with Stanley for the past fortnight and has been rewarded with a contract after impressing Beattie.
"Now I've finally signed the contract I'm really happy and I just want to start the season off with the boys and hopefully we can all contribute and do well for Accrington," Mingoia told the club's official website.
Speaking on his exit from Vicarage Road, he added: "It's not the best feeling you can have to be without a club.
"Every day you don't know what is going to happen, you hear that you might be able to go places and then the next day nothing can happen. It's not a nice feeling and to be honest I don't want to feel it again.
"I've had a couple of weeks training and I'm getting sharper but I still feel I've got a bit to go to reach my own personal standards.
"This is what I've been waiting for, to be involved somewhere, to try and help everyone out and play some games."