The Building of Y Plas actually took place over the space of about 200 years, with the front being added 1853.

At first named Greenfields, it was bought by John Edwards, attorney at law of Machynlleth. His son, (later ‘Sir’) John Edwards expanded the estate and in 1825 Sir John married widow Mrs. Harriet Herbert and had a daughter Mary Cornelia who inherited upon Sir John’s death.

In 1846 Mary Cornelia married George Henry Robert Charles William Vane Tempest 5th Marques of Londonderry. In 1884, their son, Charles, Viscount Castlereagh inherited Y Plas and became 6th Marquess, but never used Y Plas as his main home.

1886 saw of Edward, Prince of Wales visiting the area and staying at Y Plas, while, in 1911,  it was the turn of King George V, the Queen and the Prince and Princess of Wales.

Over the following years, many important events took place in and around Y Plas, the most significant for the people of Machynlleth being the fact that in 1948 Y Plas House and Grounds was given to Machynlleth by the 7th Marques.

After the bequest, Y Plas was used as council offices for many years. First by the then Machynlleth Urban District Council, (established under provisions of Local Government Act 1894).

The UDC was abolished in 1974, following local government re-organisation, and replaced by Montgomeryshire District Council and at local level by Machynlleth Town Council, who continued to sit at The Plas until the creation of the Celtica Exhibition in 1996.

Also in 1996, the Montgomeryshire District Council was abolished under local government re-organisation and replaced by Powys County Council who assumed ownership of Y Plas.
The Celtica Exhibition closed in 2006 and in 2007 Machynlleth Town Council began talks with Powys County Council over the future of The Plas.

The Town Council eventually took control on 1 April 2008, thus returning the estate to the Town in the spirit of the 1948 bequest.

          An engraving of the 6th Marquess of Londonderry, published in 1886