Her decision comes just a week after the high-profile departure of fashion director Kate Bostock.
Emmerson joined Asos in 2011 as people and services director and sat on the company's board.
Prior to joining Asos, Emmerson was group HR director at BBC Worldwide for two-and-a-half years and spent 19 years at EMI music where she held various positions including SVP HR.
Asos declined to comment after being approached by HR magazine.
