Sebastian trained in Industrial Design Engineering at Central School of Design. In 1979 he joined Wolff Olins—then Britain's leading corporate/brand identity consultancy, taking creative responsibility for key accounts. In 1982 he joined Mothercare to take responsibility for the design and development of all childcare merchandise. Mothercare soon became an international benchmark brand in its product identity design.

In 1987 he started his own merchandise and brand development consultancy, Product Identity Design, working with a variety of leading international businesses on a wide range of projects. In 1999 he merged his design studio with the Conran Group to form Conran & Partners, where Sebastian is now the Director responsible for all the product and graphics work. Over years the studio has collectively been involved with the development of thousands of products in a wide variety of markets from initial ideas through to successful merchandise.

Sebastian's technical innovations and inventions have been involved in dozens of patents in a wide variety of fields and his work has been awarded many design, marketing and innovation awards.

He has taught design at the Royal College of Art and gives frequent lectures. He is an elected member of Chartered Society of Designers, a Fellow of the Royal Society of Arts and Institute of Packaging, Trustee of the Design Museum and the Conran Foundation, and ambassador to the children's charity I CAN. He also judges and sponsors many design awards. A founding contributor to Blueprint magazine he has written numerous articles and books on design.