During his tenure at Retail Brand Alliance, Kneen designed the collections under the Casual Corner label from 2003 until 2005, prior to the brand's sale. From 2001 to 2003, he was creative director of Maska, the Italian fashion house known for its exclusive couture collection and ready-to-wear line. Prior to that, he was head designer, prêt-à-porter, for Maison Balmain, based in Paris. Kneen also led his own successful design firm and produced the Simon Kneen Collection, a line of suiting, separates, knitwear and accessories, sold through luxury apparel retailers in Italy.