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1785 February 2nd. Born in Dowlais, Merthyr Tydfil, on 2 February 1785, the eldest child of Thomas Guest, manager and part owner of the Dowlais Ironworks, and Jemima Revel Phillips of Shifnal in Shropshire. He had one brother and three sisters.
Guest was educated at Bridgnorth Grammar School and Monmouth Grammar School before learning the trade of iron making in his father's foundry at the hands of works manager John Evans. He was renowned for his ability to roll a bar of steel or cut a tram of coal as well as any of his father's workmen.
On his father's death in 1807, John inherited his share of the company and developed the business further.
1815 Guest became the sole manager of the works.
In 1817 he married Maria Rankin but their marriage was short lived, Maria dying just nine months later.
1819 Appointed Sheriff of Glamorgan 
1826 Guest was elected Member of Parliament for Honiton, Devon , holding the seat until defeated by Sir George Warrender in 1831.
In 1832 he became the first MP for Merthyr Tydfil
On 29 July 1833 he married Lady Charlotte Elizabeth Bertie, daughter of Albemarle Bertie, 9th Earl of Lindsey. They had ten children.
1834 J. J. Guest M.P. of 14 Grosvenor Square, Proprietor of the Dowlais Iron Works, Merthyr Tydvil, became an Associate member of the Institution of Civil Engineers.
His public works included a school at Dowlais, designed by Sir Charles Barry. He was a Fellow of the Royal Society and a Member of the Institution of Civil Engineers. He was the first chairman of the Taff Vale Railway
He was created a baronet in 1838
1851 Sir Josiah J. Guest living at Canford Manor House, Canford, Dorset (age 66 born Dowlais), Farming 648 acres with 27 men and 2 boys. With his wife Lady Charlotte E. Guest (age 38 born Uffington) and their children Charlotte Maria Guest (age 16 born St. Geo Han Sq, Mddx); Thomas Myrthyr Guest (age 13 born Dowlais); Montague John Guest (age 12 born St. Geo Han Sq, Mddx); Augustus Frederick (age 10 born Dowlais); Arthur Edward (age 9 born Dowlais); Mary Enid Evylen (age 7 born Dowlais); Constance Rhiannon (age 6 born Dowlais); Blanch Vere (age 3 born Great Canford). Numerous servants. 
1852 November 26th. He died leaving five sons and five daughters, the Dowlais Iron Company having become the largest producer of iron in the world. An estimated 20,000 gathered for the funeral in Dowlais.
His eldest son was Ivor Bertie Guest, 1st Baron Wimborne
1853 Obituary 
Sir John Josiah Guest, Bart., MP., was born at Dowlais, on the 2nd of February, 1785.
His grandfather, John Guest, the son of a small freeholder in Shropshire, migrated in the middle of the last century to South Wales, and in connexion with John Wilkinson, the celebrated iron-master of Bircham, they together erected at Dowlais, the first blast-furnace in the Principality, for the reduction of iron-ore, by means of pit-coal.
After the decease of Mr. John Guest, the works passed into the hands of a firm, of which his son, Thomas Guest, was a member.
In 1815, Sir John (then Mr.) Guest succeeded to the sole management, and the works, which in 1806 were considered of importance, because they produced about 5,000 tons of iron, were by the skill, perseverance and commercial enterprise of the subject of this memoir, raised in their annual power of production to 100,000 tons of pig-iron, and actually produced that quantity in 1849, when they sent into the market 75,000 tons of iron in the form of bars and rails.
Sir J. John Guest received his education principally at the Grammar School of Bridgenorth, Shropshire; the early part of his career was closely devoted to the direction of the Dowlais Ironworks, and becoming thoroughly conversant with the details of the manufacture of iron, he was fully alive to the improvements to be introduced, by a proper application of chemical and engineering knowledge, and many scientific men and members of this Institution visited the works, in furtherance of his views of ameliorating the various processes.
He tried improved blowing-engines, the substitution of raw coal for coke in the furnaces, and the use of hot-blast, with many minor alterations. He was one of the first iron-masters who undertook to roll the present heavy rails, the manipulation of which was for some time deemed nearly impracticable, and generally he spared no expense, or labour in perfecting the machinery and the processes.
The influence derived from the number of persons employed and the capital invested, joined with the general appreciation of his strong good sense and business habits, induced his return to Parliament in 1826 for the borough of Honiton, and after the passing of the Reform Bill, he represented the newly-created borough of Merthyr Tydfil, from 1832 to the period of his decease.
Although strict in enforcing subordination among the multitude of men in his employment, he was ever watchful for their interests, and sought their spiritual and temporal benefit in every way; founding places of worship, and establishing schools, whilst, during periods of mercantile depression and the visitation of disease, his charity was unbounded, and in these labours of love he was ably seconded by his wife, the Lady Charlotte Elizabeth Bertie, only sister of the Earl of Lindsey. This estimable lady, whose literary powers are as well appreciated as her general talents and acquirements in branches of knowledge not usually presenting attractive features for ladies, appears to have felt, that on arriving among a dense population of hitherto ill-educated people, speaking a peculiar dialect, and isolated by their habits and manners, few of the inhabitants of Dowlais having ever travelled twenty miles from their homes, she was called to a task of no little importance, to which she lent all the powers of her mind: she acquired the language, she visited the homes, she administered to the wants of all around her; and in thus performing her Christian duties, she extended the previously-acquired influence and power of her husband. That this influence was considerable, and was used for the good of the men, was well shown at the time of the Merthyr riots in 1831, where, but for his mediation between the men and his brother iron-masters, it is very probable that the loss of life would not have been confined to the rioters who were shot by the military, on the first outbreak of the disturbances.
For some years past, although retaining the Dowlais Works entirely in his own hands, Sir John Guest had, in consequence of failing health, chiefly resided at Canford Manor, Dorset, which he had adorned with many, specimens and curiosities, brought from Nineveh by Lady Charlotte Guest’s relative, Mr. Layard, M.P.; he however, had a great love for his early home, and his latest desire was to draw his last breath among the scenes of his childhood; where he was born and lived he desired to die and be buried.
He breathed his last on the 26th of November 1852, in his sixty-eighth year, and was followed to the grave, not only by a great number of the resident gentry of the county, and by his numerous agents and household, but by thousands of the working class, who, by their demeanour, showed how deeply they felt the loss of their benefactor and friend.
Sir John Guest received the title of Baronet in 1838; whilst his health permitted, he was very assiduous in his attention to his Parliamentary duties, and it was chiefly owing to his exertions, that the borough of Merthyr obtained the privilege of returning a Member. He joined the Institution as an Associate in 1834, and in its welfare, its in that of other scientific societies, he took considerable interest.