Argyle have been after the 20 year old for a little while and finally got their man.
Donnelly told the club's official website: "It is life-changing - it feels great not to have to worry about work and just do what I love doing.
"I used to work in a warehouse and get up at five in the morning, so I would train on a Thursday and play on Tuesdays and Saturdays.
"The manager here has told me to fight for my place in the first team and show what I can do.
"I have watched these players on TV but I feel I can do well once I get the fitness from full-time training."