Smithson was born on 18 March 1800 at Ennis, Co. Clare, Ireland (now Eire), the daughter of a theatrical manager. She made her first stage appearance in 1814 at the Crow Street Theatre, Dublin, as Albina Mandeville in Frederick Reynolds's The Will. Three years later she made her first London appearance at Drury Lane as Letitia Hardy in The Belle's Stratagem.
She had no particular success in England, but went to Paris in 1828 and 1832, first with William Charles Macready. There she aroused immense enthusiasm as Desdemona, Juliet, and as Jane in The Tragedy of Jane Shore by Nicholas Rowe. She attracted a host of admirers, among them Hector Berlioz.
Berlioz discovered her at the Odéon Theatre performing the roles of Juliet and Ophelia and immediately fell in love with her, sending her letters despite never having met her. This continued until the 1832 performance of Lélio, a sequel to his Symphonie Fantastique, when he discovered a mutual acquaintance and offered her a box of tickets. She came to the performance, realizing that the symphony was about her (as was strongly suggested by the program notes) and they married in 1833 at the British Embassy in Paris. 
At the time of her wedding, her popularity was past and she was deeply in debt, a factor believed to have strongly influenced her decision to marry. A benefit was given her, but she was coldly received. She retired from the stage. Louis Berlioz, the only child of Hector and Harriet, was born on 14 August 1834 (d. 1867). By about 1840, the marriage was failing, and Berlioz had begun affair with Marie Recio, whom he was to marry after Smithson's death. Smithson moved out of the matrimonial home on the rue Saint Vincent, Montmartre, to the rue Blanche in 1843, still financially supported by Berlioz. She was to return to her former home on the rue Saint Vincent in 1849, long after Berlioz had left it. 
Toward the end of her life, Smithson suffered from paralysis which left her barely able to move or speak. She died on March 3, 1854, at her home on the rue Saint Vincent, and was buried at the Cimetière Saint-Vincent. Berlioz later had her body reinterred at the Montmartre cemetery when Saint Vincent’s was to be destroyed. 
Portraits of Harriet Smithson
- Claude Marie Paul Dubufe (1790-1864), oil on canvas, ca. 1830, Musée Magnin, Dijon. This portrait has been copied on a large oval image painted on a vase, Paris Porcelain, manufactured by Darte, ca. 1830, Musée de la Vie romantique, Paris (gift of the Friends of the Museum, 2009).
- Raby, Peter (2004). "Smithson , Harriet Constance (1800–1854)". Oxford Dictionary of National Biography. Oxford University Press. doi:10.1093/ref:odnb/25943. Retrieved 2012-11-12. (subscription or UK public library membership required)
- Raby, P. (1982). Fair Ophelia: A Life of Harriet Smithson Berlioz. Cambridge, 216 pp, ISBN 0-521-24421-8
- Chisholm, Hugh, ed. (1911). "Smithson, Henrietta Constance". Encyclopædia Britannica (11th ed.). Cambridge University Press
- Musée de la Vie romantique, Paris