He was born in Romsey, Hampshire to a brewer. Jacob's early life is not well documented and such information as exists comes from passing mentions in his later works. He appears to have trained at the law in some manner and was a secretary to Sir William Blathwayt. Working for Blathwayt, he worked in litigation and dispensation in some fashion, although apparently in manorial courts. His first book, The Compleat Court-Keeper, of 1713 has detailed instructions for how to practically administer estate matters. He combined this with a chronological summary of statute law. Both works were financially successful.
Jacob always had an interest in contemporary poetry and the literary life, and in 1714 he wrote a farce called Love in a Wood, or, The Country Squire. This play was never produced. He persisted, however, and in 1717 he wrote a satire of Alexander Pope's The Rape of the Lock in the form of The Rape of the Smock. The poem was low and bawdy, and the next year he wrote a pornographic Tractatus de hermaphroditus.
In 1719, two works appeared by Jacob, both great successes. The first was Lex constitutionis, which was a thoroughly researched compendium of statute law, common law, and criminal law, schematized according to which powers of the executive branch of the government were involved. While the work's fame and usefulness were surpassed in a few years, Jacob's book was a well regarded analysis. The same year, he produced the first volume of the Poetical Register, with a second volume in 1720. This work provided biographies of contemporary authors as well as dead ones. These biographies have since been viewed as highly unreliable, gossipy, and hasty.
In the Poetical Register, Jacob criticized John Gay for Three Hours after Marriage (1717) and scenes that "trespass on Female Modesty" (Kilburn 547). Furthermore, he praised the phantom poet Joseph Gay (actually an author hired by Edmund Curll in his long battle against Pope and Pope's friends) for The Confederates as a satire on Gay that exposed "the false Pretence to Wit" (Kilburn 547). Jacob admired Pope and was on good terms with him, and had submitted the biography of Pope to Pope himself for correction, and he was therefore surprised when Pope attacked him for these references. Jacob did not know, apparently, that Three Hours after Marriage had been written by Gay, Pope, and John Arbuthnot, nor, apparently, that "Joseph Gay" was a creature of a polemic. In The Dunciad of 1728, Pope accused Jacob of outright stupidity and clumsiness, calling him "the Blunderbuss of Law."
In 1725, Jacob wrote The Student's Companion and called it his favorite book. It was a guide to studying law, with practical tips, reviews, and indexes. In 1729, his most famous work, nine years in the making, appeared: A New Law Dictionary. It combined a dictionary of legal practice with an abridgment of statute law, and it reached its fifth edition by the time of Jacob's death. As late as 1807, "Jacob's Law Dictionary" was still a very profitable copyright. His last work was Every Man his Own Lawyer, which outsold even the law dictionary. It was a self-help book for average citizens who might be involved in litigation.
Jacob did not marry until 1733. After his marriage, he moved to Staines, Middlesex, where he died on May 8, 1744.