The appointment, which was made without a pitch, will see Mother create press and cinema work for the brand.
Nick Jones, the owner of the Soho House Group, said: "I think Mother is fantastic and we are delighted with its creative input and enthusiasm."
The Mother partner Andy Medd added: "The Soho House Group puts creativity and quality at the very top of its agenda - that's what makes it such a distinct brand. Developing great work across the Cowshed range is going to be an absolute pleasure."
Cowshed products were first developed for use in the original Cowshed spa and bedrooms at Babington House, the Soho House-owned hotel in Somerset.
The products were redesigned by the design agency Pearlfisher in November 2005 to coincide with the launch of a standalone Cowshed spa in West London.
The product range, which was launched for the retail market at Liberty in 2001, now includes the Bullocks range for men.
It is now available at more than 50 stores in the UK, in addition to Soho House and the Scotsman Hotel in Edin-burgh. The products are also sold in Ireland, the US, Hong Kong and Australia.