The 23-year-old joined the Silkmen in September on an initial loan deal from Fleetwood, before signing a two-and-a-half-year contract at the Moss Rose in January.
Donnelly scored seven times in 32 appearances for Macc, including the second goal in a 2-1 victory over Port Vale, the Cheshire side's last winning goal in the Football League.
A statement on Macclesfield's official website read: "We would like to thank George for his efforts over the past season and wish him luck for the rest of his career."
