Hartnell is famous as the man who made London a viable twentieth century fashion centre during the inter-war years. Born to an upwardly mobile family in Streatham, in southwest London, his parents were then publicans and owners of the prophetically named Crown & Sceptre, at the top of Streatham Hill. Educated at Mill Hill School, Hartnell became an undergraduate of Magdalene College in the University of Cambridge and read Modern Languages. His main interest lay in performing, and designing productions for the university Footlights and he was noticed by the London press as the designer of a Footlights production which transferred to Daly's Theatre, London. He then worked unsuccessfully for two London designers, including the celebrated Lucile, whom he sued for damages when several of his drawings appeared unattributed in her weekly fashion column in the London Daily Sketch. In 1923 he opened his own business at 10 Bruton Street, Mayfair, with the financial help of his father and first business colleague, his sister Phyllis. The Doctor Who actor William Hartnell was his second cousin.
Thanks to his Cambridge connections, Hartnell acquired a clientele of débutantes and their mothers intent on fashionable originality in dress design for a busy social life centred on the London Season. and was considered by some to be a good London alternative to Parisian or older London dress houses. The London press seized on the novelty of his youth and gender. Although expressing the spirit of the Bright Young Things and Flappers, his designs overlaid the harder silhouettes with a fluid romanticism in detail and construction. This was most evident in Hartnell's predilection for evening and bridal gowns, gowns for court presentations, and afternoon gowns for guests at society weddings. Hartnell's success ensured international press coverage and a flourishing trade with those no longer content with 'safe' London clothes derived from Parisian designs. Hartnell became popular with the younger stars of stage and screen, and went on to dress such leading ladies as Gladys Cooper, Elsie Randolph, Gertrude Lawrence (also a client of Edward Molyneux), Jessie Matthews, Merle Oberon, Evelyn Laye and Anna Neagle. Even top French stars Alice Delysia and Mistinguett were impressed by the young Englishman's genius.
Alarmed by the lack of sales, Phyllis insisted that Norman cease his pre-occupation with the design of evening clothes and he create practical day clothes. He achieved a subtlety and ingenuity with British woollens, previously scarcely imagined in London dressmaking, yet already successfully demonstrated in Paris by Coco Chanel, who showed a keen interest in his 1927 and 1929 collections when shown in Paris. Hartnell successfully emulated his British predecessor and hero Charles Frederick Worth by taking his designs to the heart of world fashion. Hartnell specialised in expensive and often lavish embroidery as an integral part of his most expensive clothes, creating the luxurious and exclusive effect which justified the high prices. They were also created to deflect the ready-to wear copyists. The Hartnell in-house embroidery workroom was the largest in London couture and continued until his death, also producing the embroidered Christmas cards for clients and press during quiet August days, a practical form of publicity at which Hartnell was always adept. The originality and intricacy of Hartnell embroideries were frequently described in the press, especially in reports of the original wedding dresses he designed for socially prominent young women during the 1920s and 1930s, a natural extension of his designs for them as débutantes, when many wore his innovative evening dresses and day clothes.
By 1934 Hartnell's success had outgrown his premises and he moved over the road to a large Mayfair town house already provided with floors of work-rooms at the rear to Bruton Mews. The first floor salon was the height of modernity, like his clothes and the glass and mirror-lined Art Moderne space was designed by the innovative young architect Gerald Lacoste (1909–1983). The interiors of the large late 18th-century town house are now protected as one of the finest examples of art-moderne pre-war commercial design in the UK. The timeless quality of Lacoste's designs was the perfect background for each new season of Hartnell designs, created for aristocratic British women of all ages and worn by most of the famous theatre and film stars of their day, including Vivien Leigh, Gertrude Lawrence, Merle Oberon, Ann Todd, Evelyn Laye, Anna Neagle and trans-Atlantic stars such as Marlene Dietrich, Elizabeth Taylor and Linda Christian. At the same time, Hartnell moved into the new building, he acquired a week-end retreat, Lovel Dene, a Queen Anne cottage in Windsor Forest, Berkshire. This was extensively re-modelled for him by Lacoste. London life was based in The Tower House, Park Village West Regent's Park, also re-modelled and furnished with a fashionable mixture of Regency and modern furniture.
In 1935 Hartnell received the momentous first royal commands, inaugurating four decades of his worldwide fame and success in providing clothes for the ladies of the British Royal Family. Lady Alice Montagu-Douglas-Scott, the future Princess Alice, Duchess of Gloucester, a daughter of the Duke of Buccleuch, approached Hartnell to design her dress and those of her bridesmaids for her marriage to Prince Henry, Duke of Gloucester, third son of King George V. Two bridesmaids were Princess Elizabeth and Princess Margaret, daughters of the Duke and Duchess of York (the future King King George VI and his consort Elizabeth). Both George V and Queen Mary approved the designs, the latter also becoming a client. The future Queen Elizabeth, then a client of Madame Handley-Seymour, who had made her wedding dress in 1923, accompanied her daughters to the Hartnell salon to view the fittings and met the designer for the first time.