He apprenticed with a hand - loom reed maker, but is said to have returned home within a month claiming to have mastered the business. He designed a metal substitute for the natural reed that proved popular enough for him to sell throughout England. After traveling the country, making and fitting wire reeds, he returned to Bury and, on 29 June 1725, both he and his brother, William, married Bury women. John's wife was Anne Holte. His daughter Lettice was born in 1726, and his son Robert in 1728.

In Bury he continued to design improvements to textile machinery; in 1730 he patented a cording and twisting machine for worsted.

In 1733, he received a patent for his most revolutionary device: a "wheeled shuttle" for the hand loom. It greatly accelerated weaving, by allowing the shuttle carrying the weft to be passed through the warp threads faster and over a greater width of cloth. It was designed for the broad loom, for which it saved labor over the traditional process, needing only one operator per loom (before Kay's improvements a second worker was needed to catch the shuttle).

Kay always called this invention a "wheeled shuttle", but others used the name "fly - shuttle" (and later, "flying shuttle") because of its continuous speed, especially when a young worker was using it in a narrow loom:

a speed which cannot be imagined, so great that the shuttle can only be seen like a tiny cloud which disappears the same instant.

—Roland de la Platière, Encyclopédie Méthodique (1785)

In July 1733, Kay formed a partnership in Colchester to begin fly - shuttle manufacturing. No industrial unrest was anticipated, this being the first device of the modern era to significantly enhance productivity. But by September 1733 the Colchester weavers, were so concerned for their livelihoods that they petitioned the King to stop Kay's inventions.

The flying shuttle was to create a particular imbalance by doubling weaving Productivity without changing the rate at which thread could be spun, disrupting spinners, and weavers alike.

Kay tried to promote the fly - shuttle in Bury, but couldn't convince the woollen manufacturers that it was sufficiently robust; he spent the next two years improving the technnology, until it had several advantages over the device specified in the 1733 patent. (Which was to be one of his difficulties in the coming patent disputes.)

In 1738 Kay went to Leeds, where his problem had become royalty collection. (The annual license fee was 15 Shillings per shuttle.) He continued to invent, patenting some machines in the same year, though these were not taken up industrially.

Kay (and, initially, his partners) launched numerous patent infringement lawsuits, but if any of these cases were successful, compensation was below the cost of prosecution. Rather than capitulate, the manufacturers formed "the Shuttle Club", a syndicate which paid the costs of any member brought to court; their strategy of patent piracy and mutual indeminification nearly bankrupted Kay.

In 1745, he and Joseph Stell patented a machine for cloth ribbon weaving, which they anticipated might be worked by water wheel, but they were unable to advance their plans because of Kay's legal costs. Impoverished and harassed, Kay was compelled to leave Leeds, and he returned to Bury. Also in 1745, John's twelfth (and final) child, William, was born.

Kay remained inventive; in 1746 he was working on an efficient method of salt production, and designing improvements to spinning technology - but that made him unpopular among Bury spinners Also, fly - shuttle use was becoming widespread in weaving, increasing cotton yarn demand and its price - and Kay was blamed.

He had suffered violent treatment in England, but he did not leave the country on that account, but because of his inability to enforce (or profit from) his patent rights. Trudaine's Bureau de Commerce was known to support textile innovations (and would later actively recruit immigrant inventors). Probably encouraged by the prospect of state support, in 1747, Kay left England for France (where he had never been before, and did not speak the language).

He went to Paris, and throughout 1747 negotiated with the French Government (in English) to sell them his technology.

Denied the huge lump sum he wanted, Kay finally agreed to 3,000 livres plus a pension of 2,500 livre, (annually from 1749) in exchange for his patent, and instruction in its use (to the manufactures of Normandy). He retained the sole rights to shuttle production in France, and brought three of his sons to Paris to make them. Although wary of entering the manufacturing provinces (because of his experiences with rioting weavers in England) he was prevailed upon to do so.

At one time, the French authorities may have discouraged his communication with England, but Kay wrote about the unanticipated use of his technology in England to the French government:

My new shuttles are also used in England to make all sorts of narrow woollen goods, although their use could have been more perfect had the weavers consulted me

—John Kay, Letter in the French Archives nationales

The beginning of mechanization in French textile production is traditionally dated to 1753, with the widespread adoption of the flying shuttle there. Most of these new shuttles were copies, not made by the Kays. John Kay unsuccessfully tried to enforce his manufacturing monopoly, and began to quarrel with the French authorities, briefly returning to England, in 1756. (It is said that he was in his Bury home in 1753 when it was vandalized by a mob - and that he narrowly escaped with his life, but this is probably a 19th century tale based on earlier Colchester riots. Kay was probably in France throughout the early 1750s.)

He found his prospects in England unimproved; by 1758 he was back in France, which became his adopted country, though he was to visit England at least twice more. In the winter of 1765 - 66 he appealed to the Royal Society of Arts to reward him for his inventions, and exhibited his card - making machine for them. The Society could find no-one who understood the shuttle, and there was a breakdown in correspondence, so that no award was ever made. He was in England again in 1773, but returned to France in 1774 having lost his pension (at aged 70).

His offer to teach pupils if the pension were restored was not taken up, and he spent his remaining years developing and building machines for cotton manufactures in Sens and Troyes. Though he was busy with engineering and letter writing until 1779, he received only 1,700 livres from the French state over these five years, reaching a state of penury in March 1778 before receiving his final advance (to develop yet more machinery).

His last known letter (1779 - 06 - 08) listed his latest achievements for the Intendant de Commerce, and proposed further inventions. But since these were never made, and no more is heard of the 75 year old Kay, it is believed that he must have died later in 1779.

John Kay's son, Robert, stayed in England, and in 1760 developed the "drop - box", which enabled looms to use multiple flying shuttles simultaneously, allowing multicolor wefts. His son John ("French Kay") had long resided with his father in France. In 1782 he provided an account of his father's troubles to Richard Arkwright, who sought to highlight problems with patent defense in a parliamentary petition.

Ford Madox Brown portrayed Kay and his invention in a mural painting in Manchester Town Hall.

In Bury, Kay has become a local hero: there are still several pubs named after him, as are the Kay Gardens.

In the 1840s, Thomas Sutcliffe (one of Kay's great - grandsons) campaigned to promote a Colchester heritage for Kay's family. In 1846 he unsuccessfully sought a parliamentary grant for Kay's descendants (in compensation for his ancestor's treatment in England). He was inaccurate in the details of his grandfather's genealogy and story, and his "Fanciful and Erroneous Statements" were discredited by John Lord's detailed examination of primary sources.