Records go back to 1348. From the Guild‘s website: "In its widest sense mercery could describe all merchandise, although in London the term evolved to mean the trade specifically in luxury fabrics, such as silk, linen, hemp-cloth and fustian, and in a large variety of miscellaneous 'piece goods' such as bedding, headwear, ribbons, laces and purses."
At Queen's Theatre you can see a Mercers' maiden. This is the symbol of the Mercers' Company and was used to mark their property. It can be seen on various buildings across London. Richard R gives more information.