Fernandes replaces Mark Palmer, who will remain at the brand in a 'non-executive capacity'. He rejoined Green & Black's in December 2007, having left after its acquisition by Cadbury in 2005.
Fernandes, who prior to Green & Black's was Cadbury's category innovation manager for global candy, will oversee marketing in the UK, the US, Ireland, Canada, Australia and New Zealand.
Her previous marketing roles have included stints at Nestle and Kellogg.
In late 2007, Green & Black's carried out a managerial restructure in which it scrapped the position of UK marketing director, occupied at the time by Carol Welch.
The brand said it would increase its UK marketing spend in 2008 by 25% to £7m, and use outdoor and digital marketing for the first time.