John's father died in 1802, leaving his family with small means. John went to work at the age of 9 as a bobbin-winder. His mother remarried Thomas Mercer of Eggsyke and they built a new house known as Dean Cottage. Within a few months, they both died (1810) leaving a small son named Billy. Joseph Clayton, brother of Mrs. Mercer, was named guardian of the child and John went to live with them. John became a militiaman and earned the name "Awkward John" and was transferred to the band.

When he became a bit older he began rooming with a Methodist family named Wolstenholme, John married Mary Wolstenholme from that family (the Wolstenholme's lived at Cliffe across from the Dog and Otter Pub.) They became deeply religious and joined the Wesleyans.

His first inevention was a formula for red ink for which he received the large sum (in those days) of 10,000 pounds in 1844. In 1850 he patented the process of Mercerization which threads cotton fibers with caustic soda producing a lusterous swelling of the fibers. He had a factory on Dean Brook.

John Mercer was a generous man who donated much to the community, including Mercer Hall in Great Harwood and Mercer Park in Clayton-le-Moors. A large memorial clock in the center of Great Harwood is dedicated to him. He died on 11/30/1866 of a "painful disease" thought to have been brought on from a fall in a stream the year before, which caused a severe cold at Oakenshaw House in Clayton-le-Moors and is buried at St Bartholomew's Church in Great Harwood.