Day had been with the business for seven years and was previously sales director at Grants of St James's Wines.
While at Ehrmanns he set up the Fairtrade Wine Committee to raise consumer and trade awareness of Fairtrade wine in the UK. He directed the formation of the annual Fairtrade wine competition and tasting.
Day also worked closely with Arniston Bay, which has now moved to Percy Fox after the Company of Wine People ended its joint venture with Ehrmanns.
Ehrmanns chairman Peter Dauthieu said: "I would like to personally thank Nick for his significant contribution to the Ehrmanns business over the past seven years. While I am saddened by his decision to move on, we all wish him the very best with his plans for the future."