The second half of the 19th century was a period of growth for the town.. Until 1914 the town held its own but after the First Word War there began a slow but steady decline in its fortunes.

In 1911 the two quarries, Graiglwyd and Penmaen amalgamated to form the Penmaenmawr Welsh Granite Co. Set making had come to an end by the 1930s and production concentrated on aggregate for rail ballast, road surfacing and concrete. Unfortunately, this resulted in the creation of fine dust which settled on the town and brought complaints from residents and visitors. At the beginning of the 1930s a thousand men were employed in the Quarry but changes in manufacturing processes based on American models resulted in a big reduction in manpower by the 1950s - locomotives were replaced by lorries and conveyors replaced the wagon carrying inclines.

  

By the end of the 1970s both jetties were defunct. By the end of the century less than thirty men were employed although high production continued especially for rail ballast. Since the 1960s ownership had changed twice. Many of the old buildings were demolished in the 1990s and much of the spoil covered sea facing slopes landscaped.