The role will cover both Elle and the recently launched bi-annual fashion magazine Elle Collections.
Tony Long, Hachette's director of sales, said Greaves' role had become much broader, taking in events and digital plans, and the added resource was needed to maintain focus on client relations.
Whitmarsh joined Condé Nast for the launch of Easy Living, initially as ad manager, and had previously been ad manager on Hello!.
Condé Nast said it would not replace her directly, but was looking for a new ad manager to report to associate publisher Jane Lewis.
Hachette has also made Lee Bailey advertising director for Real Homes after eight months of maternity cover for Alex Heath, the magazine's ad manager, and Michele McQueen, group ad director on both Real Homes and Elle Decoration.
Bailey will report to Long until McQueen returns in May next year.