Carr worked with Sinclair chief executive at Hozelock, as did Sinclair's recently appointed marketing director Simon McArdle.
Trade managing director John Tugman has left. Retail managing director Danny Adamson left this spring as the growing media company moved away from a divisional structure.
Carr was at Spear & Jackson before he joined Hozelock in 2008.
Garden centre sales director Steve Davis has also left Sinclair, to join Gabriel Ash.