Sir Josiah John Guest, 1st Baronet (2 February 1785 – 26 November 1852), known as John Josiah Guest, was a British engineer, entrepreneur and politician.

Guest was born on 2 February 1785 in Dowlais, Merthyr Tydfil, Wales. He was the son of Thomas Guest (d. 1807), a partner in the Dowlais Iron Company, and Jemima Revel Phillips. Guest was educated at Bridgnorth Grammar School and Monmouth School.^[1]

After attending school, he learned the trade of ironmaking in his father's foundry at the hands of the works manager, John Evans. Guest 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.^[1] Upon his father's death in 1807, Guest inherited his father's share of the company and developed the business, becoming sole owner of the works in 1815. By the time of his death in 1852, the Dowlais Iron Company had become the largest producer of iron in the world.^[2]

Guest was elected in 1825 as Member of Parliament for Honiton, Devon, holding the seat until the 1831 general election during the Reform Crisis.^[2] In 1832 he became the first MP for Merthyr Tydfil as a Whig,^[2] and in 1837 he launched the campaign to have the town incorporated.^[3]

In 1838, Guest was created a baronet, of Dowlais in the County of Glamorgan.^[2][4] After his death in 1852, Guest was succeeded by his eldest son, who was elevated to the peerage in 1880 as Baron Wimborne, of Canford Magna in the County of Dorset, on Disraeli's initiative.^[5]

On 11 March 1817, Guest married Maria Rankin but their marriage was short-lived, Maria dying just nine months later in January 1818. On 29 July 1833, he subsequently married Lady Charlotte Elizabeth Bertie (1812–1895), daughter of Albemarle Bertie, 9th Earl of Lindsey.

Together, they had five sons and five daughters, including:

• Charlotte Maria Guest (1834–1902), who married Richard Du Cane (d. 1904), brother of Edmund Frederick Du Cane.^[7]
• Ivor Bertie Guest (1835–1914), who married Lady Cornelia Henrietta Maria Spencer-Churchill (1847–1927), daughter of John Spencer-Churchill, 7th Duke of Marlborough^[2]
• Katharine Gwladys Guest (1837–1926), who married the Reverend Frederick Cecil Alderson (d. 1907), son of Sir Edward Hall Alderson^[7]
• Thomas Merthyr Guest (1838–1904), who married the writer Lady Theodora Grosvenor (1840–1924), daughter of Richard Grosvenor, 2nd Marquess of Westminster^[7]
• Montague John Guest (1839–1909), a Liberal politician, who never married.^[8]
• Augustus Frederick Guest (1840–1862), who died aged 21.^[7]
• Arthur Edward Guest (1841–1898), a Conservative politician, who married Adeline Mary Chapman (d. 1931)^[7]
• Mary Enid Evelyn Guest (1843–1912), who married Sir Austen Henry Layard (1817–1894).^[9]
• Constance Rhiannon Guest (1844–1916), who married Hon. Charles George Cornwallis Eliot (1839–1901), youngest son of Edward Eliot, 3rd Earl of St Germans^[7]
• Blanche Vere Guest (1847–1919), who married Edward Ponsonby, 8th Earl of Bessborough (1851–1920).^[7]

Guest died in 1852 and was buried in an iron coffin under a red granite slab in the local St John's Church, which had been built for him in 1827.^[10]

After his death, his widow married Charles Schreiber (1826–1884) in 1855.^[9] Schreiber was an academic who had been Ivor's tutor in 1852. He was known for being a fine arts collector and Conservative Party politician who sat in the House of Commons between 1865 and 1884.^[11]