Between 1956 and 1963 she studied jersey and knitwear design and manufacturing while working for Jacqmar and then Jaeger and in 1961 started her own line of clothing under the label Jane & Jane. In 1966 she founded Jean Muir Ltd. with her husband.

Muir particularly liked working in soft, flowing fabrics: jersey, wool crêpe, silk satin, and soft leathers and suedes. Fluidity was particularly important to Muir who ensured comfort in all her clothes. Muir modeled her own toiles to ensure everything fit and flowed. She believed the secret to designing was primarily in understanding the female anatomy and took an exacting, mathematical approach to her designs. Muir had a reputation for perfectionism and exacting standards in all aspects of production. Although her character seemed strict and disciplined, her clothes were not. Her fashions stayed away from restrictive interfaced fabrics, structured tailoring and underpinnings. Her clothes were emancipated, flowing and had a timeless quality to them, although her collections would never be mistaken for anything that was at the cutting edge of style.