Josiah John Guest (1785-1852) was the grandson of a Staffordshire brewer, farmer and coal dealer who became manager of the Dowlais Ironworks in 1767. He inherited this vast industrial concern in 1807 and ensured that it overtook Cyfarthfa as the largest ironworks in the town and indeed the world, rolling many hundreds of miles of railway lines to supply the 'Railway mania' of the early 19th century.

He married twice, firstly to Maria Rankin who died very young, and secondly to the popular Lady Charlotte Guest. He was Merthyr's first M.P. in 1832, and pioneered schooling and library provision for his workers.

Paradoxically, he was also a champion of the unpopular 'truck' system of paying tokens instead of wages to be redeemed in company shops, and provided inadequate housing for his workers, making Dowlais a particularly crowded and insanitary place to live in his time.