Robinson was born in Bristol, England to Nicholas Darby, a naval captain, and his wife Hester (née Vanacott). She was baptised 'Polle(y)'. In her memoirs, Robinson gives her birth in 1758 but the year 1757 seems more likely according to recently published research. Her father deserted her mother and took on a mistress when Robinson was still a child. Darby died in the Russian naval service in 1785. Robinson, who at one point attended a school run by the social reformer Hannah More, came to the attention of actor David Garrick.
Hester Darby encouraged her daughter to accept the proposal of an articled clerk, Thomas Robinson, who claimed to have an inheritance. Mary was against this idea; however, after being stricken ill, and watching him take care of her and her younger brother, she felt that she owed him, and she did not want to disappoint her mother. After the early marriage, Robinson discovered that her husband did not have an inheritance. He continued to live an elaborate lifestyle, however, and had multiple affairs that he made no effort to hide. Subsequently, Mary supported their family. After her husband squandered their money, the couple fled to Talgarth, Breconshire (where Robinson's only daughter, Mary Elizabeth, was born in November). Eventually her husband was imprisoned for debt in the Fleet Prison where she lived with him for many months. While it was common for the wives of prisoners to live with their husbands while indebted, children were usually sent to live with relatives to keep them away from the dangers of prison. However, Robinson was deeply devoted to her daughter Maria, and when her husband was imprisoned, Robinson brought the 6-month-old baby with her. It was in the Fleet Prison that Robinson’s literary career really began, as she found that she could publish poetry to earn money, and to give her an escape from the harsh reality that had become her life. During this time, Mary Robinson found a patron in Georgiana Cavendish, Duchess of Devonshire, who sponsored the publication of Robinson's first volume of poems, Captivity.
After her husband obtained his release from prison, Robinson decided to return to the theatre. She launched her acting career and took to the stage, playing Juliet, at Drury Lane Theatre in December 1776. Robinson was best known for her facility with the 'breeches parts', her performances as Viola in William Shakespeare'sTwelfth Night and Rosalind in As You Like It won her extensive praise. But she gained popularity with playing in Florizel and Perdita, an adaptation of Shakespeare, with the role of Perdita in 1779. It was during this performance that she attracted the notice of the young Prince of Wales, later King George IV of the United Kingdom. He offered her twenty thousand pounds to become his mistress.
With her new social prominence, Robinson became a trend-setter in London, introducing a loose, flowing muslin style of gown based upon Grecian statuary that became known as the Perdita. It took Robinson a considerable amount of time to decide to leave her husband for the Prince, as she did not want to be seen by the public as that type of woman. Throughout much of her life she struggled to live in the public eye and also to stay true to the values in which she believed. She eventually gave in to her desires to be with a man whom she thought would treat her better than Mr. Robinson. However, the Prince ended the affair in 1781, refusing to pay the promised sum. "Perdita" Robinson was left to support herself through an annuity promised by the Crown (but rarely paid), in return for some letters written by the Prince, and through her writings.
Mary Robinson, who now lived separately from her husband, went on to have several love affairs. Her relationship with Tarleton began on a bet, but lasted 15 years. They had no children, although Robinson had a miscarriage. However, in the end, Tarleton married Susan Bertie, an heiress and an illegitimate daughter of the young 4th Duke of Ancaster, and niece of his sisters Lady Willoughby de Eresby and Lady Cholmondeley. In 1783, Robinson suffered a mysterious illness that left her partially paralysed. Biographer Paula Byrne speculates that a streptococcal infection resulting from a miscarriage led to a severe rheumatic fever that left her disabled for the rest of her life.
From the late 1780s, Robinson became distinguished for her poetry and was called "the English Sappho". In addition to poems, she wrote eight novels, three plays, feminist treatises, and an autobiographical manuscript that was incomplete at the time of her death. Like her contemporary Mary Wollstonecraft, she championed the rights of women and was an ardent supporter of the French Revolution. She died in poverty at Englefield Cottage, Englefield Green, Surrey, 26 December 1800, aged 44, having survived several years of ill health, and was survived by her daughter, Maria Elizabeth (1774-1818), who was also a published novelist. Administration of her estate was granted to her husband Thomas Robinson from whom she had long been separated and who in 1803 inherited a substantial estate from his half-brother William. One of Robinson’s dying wishes was to see the rest of her works published. She tasked her daughter, Maria Robinson, with publishing most of these works. She also placed her Memoirs in the care of her daughter, insisting that she publish the work. Maria Robinson published Memoirs just a few months later.
