Cotman was born in Norwich, England, the eldest son of a prosperous silk merchant and lace dealer, and was educated at the Free Grammar School. He showed a talent for art from an early age and would go out on frequent drawing trips into the surrounding countryside. Cotman's father intended him to go into the family business but instead, intent on a career in art, he moved to London in 1897 - 98; He made a living, initially, through commissions from print-sellers. He came under the patronage of Dr. Thomas Monro, whose house was also a studio and meeting place for local artists. Here Cotman made the acquaintance of J M W Turner, Peter de Wint and Thomas Girtin - the latter, in particular, becoming a very influential figure in his artistic development. He joined a sketching club started by Girtin, and went on drawing expeditions to Wales and Surrey.

In 1800, at the age of 18, Cotman had his first work exhibited at the Royal Academy and was also awarded an honorary palette by the Society of Arts. He continued to exhibit at the Academy until 1806, and also went on several extended drawing trips through England and Wales.

In 1807 he returned to his home city, Norwich and, joined the "Norwich Society of Artists", exhibiting many of his works there in 1808. In 1811 he became president of the society.

In 1809, Cotman married Ann Mills, daughter of a local farmer. They went on to have five children together. His main living came from the teaching of art and one of his students, local antiquary Dawson Turner, became a good friend, introducing him to many pupils and collaborating on one of his books. Apart from painting and drawing, Cotman made many etchings of old buildings and sepulchral brasses in Norfolk which were published in 4 volumes between 1811 and 1819. He also made several journeys to Normandy in France, between 1817 - 20, and the engravings produced were also published in 2 volumes in 1822.

From 1812 to 1823, Cotman resided in Yarmouth and, being on the coast, he was able to study shipping and master the form of waves - it's no surprise that some of his finest marine pieces date from this time. However, he returned to Norwich in 1824 in order to better his financial position, and showed work from 1823 to 1825 at the Society of Artists' annual exhibitions. He was a collector of prints, books and armour, and also had many models of ships to help him with his compositions.

In 1825, Cotman became an Associate of the Royal Society of Painters in Watercolours and was a frequent exhibitor until 1839. However he was driven to despair by his constant financial struggles.

In January 1834, Cotman's luck changed for the better, for he was appointed Professor of Drawing at King's College School in London - partly on the recommendation of J M W Turner. In 1836, his son, Miles Edmund was also appointed to assist him. Of his pupils mention must be made of a certain Dante Gabriel Rossetti.

In London Cotman was friends with a number of artists including James Stark, George Cattermole, Samuel Prout, Cornelius Varley and others. In 1836, he became an honorary member of the Institute of British Architects. In 1838, all of his etchings were published by Henry George Bohn including "Liber studiorum".

Cotman died in July 1842, and was buried in the cemetery behind St. John's Wood chapel. Subsequently, all of his works and his collection of prints and books were sold by auction at Christie's, realising just over £525 - a relatively paltry sum.

His sons, Miles Edmund Cotman and John Joseph Cotman, also became painters of note. Cotman's name is used as a trademark by Winsor & Newton for a range of artist's watercolour materials.

Cotman worked in oils, watercolour, pencil and chalk, as well as producing many hundreds of etchings. His work can be found in the UK at the Castle Museum and Art Gallery in Norwich (well over 2000 pieces), Tate Gallery, British Museum and Victoria & Albert Museum in London, the Fitzwilliam Museum in Cambridge, City Art Gallery in Leeds and other regional centers. In the USA, Cotman is represented at the Yale Center for British Art in New Haven, Connecticut, and other galleries around the country.