James Jennings was the son of a village shopkeeper, John Jennings, and of his wife Elizabeth Fear. He was born in Huntspill, Somerset, where he was educated locally at North Petherton school. He possibly had a sister named Sarah Jennings born around 1781.
In 1786 he was apprenticed to a Bristol Apothecary and remained for several years. Whilst in Bristol he published, in a Bristol paper, a series of essays, under the title of the "Speculator" and several of his poetical pieces appeared in the European Magazine. In 1794, he published The Times, a satire.
Jennings married Charlotte Sawier, probably the only daughter of Southey's landlady Mary Sawier, shortly before moving to London towards the end of 1795. However, after Jennings' health became greatly impaired, and at the solicitation of his father, he returned to Huntspill in 1801 to work in his family's grocery shop. His father retired soon after, but James Jennings remained in Huntspill until the mid-1810s when economic depression led to the failure of the business. During this period he was by no means idle, continuing his literary pursuits. He contributed to the Monthly Magazine from 1807, and occasionally to the provincial newspapers. In 1810 he published a volume of poems consisting of "The Mysteries of Mendip, the Magic Ball, Sonnets," and in 1814, "The Prospects of Africa, and other Poems,".
In 1807, Jennings' wife died leaving him a widower with four children. His second wife was Miss Rouquet, a daughter of a respectable clergyman.
In 1817, he returned to London where he worked as a professional writer, with some support from a wealthy baker, Sir William Paxton. His works included the Family Cyclopaedia (1821), Observations on Some of the Dialects of the West of England (1825) and Ornithologia (also known as The Birds) which is a poem in two parts (1828). He also founded the short-lived Metropolitan Literary Institution in 1823 and was editor of the Metropolitan Literary Journal (1824).
In 1794, whilst in Bristol, Jennings met Southey and Coleridge, and although they were not close friends, he and Southey corresponded and remained in contact until c.1828, (the correspondence has not survived). Jennings was a great admirer of Southey's writing, but the admiration was apparently not returned. Southey nicknamed him 'poor Trauma' and 'the traumatic poet', though he admired Jennings's 'moral character'.
Jennings shared Southey's interest in educational methods, and so in collaboration with the local rector at Huntspill, he established a school conducted on Lancaster and Bell's monitorial system in 1813. Jennings included anecdotes of Southey and Coleridge's early careers in the Metropolitan Literary Journal (1824).