Lucienne Day joined the Royal College of Art in 1937, by which time the failings of the college with regard to industrial design - lamented by her future husband Robin Day - had to some extent been removed. She graduated in 1940 (her graduation show providing opportunity for the first cooperation with Robin), and began to design fabrics for Sekers. However the discovery that she could not expect to be credited for her work at the company caused her to leave. She and Robin married in 1942.

After the war she designed fabrics for the clothing industry, since this was less subject to post-war restrictions than the interior design side of the business - where her real interests lay. The 1951 Festival of Britain provided a showcase for her work in this area, in particular an abstract furnishing fabric, Calyx, which was subsequently manufactured by Heal's. This was not her first work for Heal's, but it kick-started their association. Subsequently many of her designs were commissioned by the company.

The development of her style can be traced in Lesley Jackson's Robin and Lucienne Day, published in 2001.