He started work in Llanelli in 1859
In 1868 he joined J & H Gwynne & Co in London where he stayed for eleven years rising to general manager. The company manufactured centrifugal pumps, steam engines and electrical generating dynamos. The steam engines were coupled to the pumps and dynamos.
It was during his time at Gwynnes that he established his many connections with the leading engineers and shipbuilders of the day.
In 1880 William Henry resigned and started his own company. He found suitable premises in York Street in Lambeth.
Legend has it that William Henry Allen was on his way back to London on the 12th of November 1893 after visiting the Midlands looking for new premises. While he was sitting on the old Bedford station waiting for his train back to London he noticed a For Sale hoarding opposite the station on the other side of the tracks.
The piece of land for sale was 20 acres in size and was part of the Queens Park Estate which was owned by the Whitbread family. A deal was agreed before the day was out and a 10% deposit paid and William Henry returned to London.
The legal formalities were completed in five weeks and William Henry was cutting the first sod on the 20th of January 1894, only 11 weeks after first seeing the site.
Although this was the start of Allens in Bedford it was not the start of the company.