McCormack has penned a four-year deal with Felix Magath's side, with an option of another year, following their relegation from the Premier League.
Goal understands McCormack shunned interest from the top flight, believed to have been from QPR, to sign for the Cottagers.
The 27-year-old scored an impressive 28 goals in the Championship with Leeds last season, finishing as the division's top goalscorer ahead of the likes of Jordan Rhodes and Watford striker Troy Deeney.
Leeds owner Massimo Cellino had originally been reluctant to sell McCormack, with the club issuing a statement on July 3 insisting that it was their intention to keep their best players as they looked to build a successful squad for the 2014-15 campaign.
However, the Italian finally sanctioned the sale with McCormack set to link up with Magath's side at their training camp in Scotland, where the club will contest a pre-season fixture with Motherwell at Fir Park on Saturday having beaten East Fife 3-0 in their first friendly of the campaign on July 5.