Green was out of contract at Mansfield and, despite the best efforts of the League Two club to retain his services, Blues have fought off plenty of competition to land the 26-year-old.
Green fired 56 goals for the Stags over the previous two campaigns, including 27 last season as he spearheaded the club's charge back into the Football League.
The frontman started out with Newport before signing for Cardiff in 2007 although he spent the majority of his time there out on loan.
A move to Torquay quickly followed in 2008 but his stay at Plainmoor was only to last one term before he linked up with Oxford on a season-long loan, culminating in a permanent move to the Kassam Stadium in the summer of 2010.
Further loan spells followed at Cheltenham and then Mansfield, who snapped up his services on a permanent deal last summer after a stunning initial campaign which produced 29 goals.