He was born in Liverpool, where his father, a market gardener, kept a public house called the Bowling Green at Mount Pleasant. Roscoe was left school at the age of twelve, having learned all that his schoolmaster could teach. He assisted his father in the work of the garden, but spent his leisure time on reading and study. "This mode of life," he says, "gave health and vigour to my body, and amusement and instruction to my mind; and to this day I well remember the delicious sleep which succeeded my labors, from which I was again called at an early hour. If I were now asked whom I consider to be the happiest of the human race, I should answer, those who cultivate the earth by their own hands." At fifteen he began to look for a suitable career. A month's trial of bookselling was unsuccessful, and in 1769 he was articled to a solicitor. Although a diligent student of law, he continued to read the classics, and made the acquaintance with the language and literature of Italy which was to dominate his life.
His poem, Mount Pleasant, was written when he was sixteen, and together with other verses, now forgotten, won the esteem of good critics. In 1774 he went into business as a lawyer, and in 1781 married Jane, second daughter of William Griffies, a Liverpool tradesman; they had seven sons and three daughters. Roscoe had the courage to denounce the African slave trade in his native town, where much of the wealth came from slavery. He wrote the Wrongs of Africa (1787–1788), and entered into a controversy with an ex-Roman Catholic priest, who tried to justify the slave trade through the Bible. Roscoe was also a political pamphleteer, and like many other Liberals of the day hailed the promise of liberty in the French Revolution.
Meanwhile he had pursued his Italian studies, and had carried out research, which resulted in his Life of Lorenzo de' Medici, which appeared in 1796, and gained him a reputation among contemporary historians. It was often reprinted, and translations in French, German and other languages show that its popularity was not confined to Britain. Fabroni, who had intended to translate his own Latin life of Lorenzo, abandoned the idea and persuaded Gaetano Mecherini to undertake an Italian version of Roscoe's work.
In 1796 Roscoe gave up legal practice, and toyed with the idea of going to the bar. Between 1793 and 1800 he paid much attention to agriculture, and helped to reclaim Chat Moss, near Manchester. He also succeeded in restoring to good order the affairs of a banking house in which his friend William Clark, then resident in Italy, was a partner. This led to his introduction to the business, which eventually proved disastrous. His translation of Luigi Tansillo's Nurse appeared in 1798, and went through several editions. It is dedicated in a sonnet to his wife, who had practised the precepts of the Italian poet.
The Life and Pontificate of Leo the Tenth appeared in 1805, and was a natural sequel to his previous work of history. The new work, whilst it maintained its author's fame, did not meet with so favourable a reception as the Life of Lorenzo. It was frequently reprinted, and the insertion of the Italian translation in the Index did not prevent its circulation even in the papal states. Roscoe was elected member of parliament for Liverpool in 1806, but the House of Commons was not for him, and at the dissolution in the following year he stood down. The commercial troubles of 1816 brought into difficulties the banking house with which he was connected, and forced the sale of his collection of books and pictures. It was on this occasion that he wrote the fine Sonnet on Parting with his Books.
Dr SH Spiker, the king of Prussia's librarian, visited Roscoe at this difficult time. Roscoe said he still desired to write a biography of Erasmus but lacked both leisure and youth. The project was never carried out. After five years struggling to discharge the liabilities of the bank, the action of a small number of creditors forced the partners into bankruptcy in 1820. For a time Roscoe was in danger of arrest, but ultimately he received honourable discharge. On the dispersal of his library, the volumes most useful to him were secured by friends and placed in the Liverpool Athenaeum. The sum of £2500 was also invested for his benefit.
Having now resigned commercial pursuits entirely, he found a pleasant task in the arrangement of the great library at Holkham, the property of his friend Thomas Coke. In 1822 he issued an appendix of illustrations to his Lorenzo and also a Memoir of Richard Robert Jones of Aberdaron, a remarkable self-taught linguist. The year 1824 was memorable for the death of his wife and the publication of his edition of the works of Alexander Pope, which involved him in a controversy with William Lisle Bowles. His versatility was shown by the appearance of a folio monograph on the Monandrian Plants, which was published in 1828. The last part came out after his recovery from a stroke.
Roscoe showed considerable moral courage as well as devotion to study. He had many friends. Posterity is not likely to endorse the verdict of Horace Walpole, who thought Roscoe the best of our historians, but his books on Lorenzo de' Medici and Pope Leo X remained important contributions to historical literature. In addition to these, Roscoe wrote tracts on penal jurisprudence, and contributed to the Transactions of the Royal Society of Literature and of the Linijean Society. The first collected edition of his Poetical Works was published in 1857, and is sadly incomplete, omitting, with other verses known to be from his pen, the Butterfly's Ball, a fantasy, which has charmed thousands of children since it appeared in 1807. Other verses are in Poems for Youth, by a Family Circle (1820).
The Life by his son Henry Roscoe (2 vols., London, 1833) contains full details of Roscoe's career, and there are references to him in the Autobiographical Sketches of De Quincey, and in Washington Irving's Sketch Book.