Educated at St John's college, Cambridge. Did not take active part in business but became a Banker and then a farmer at estate of Gorddinog which he inherited. He founded the Welsh Black Cattle Society in 1873, and published 'Notes on Black Cattle' in 1885. Was defeated by Lloyd George in 1900 election at the Caernavon election. His son, Major Eric James William Platt (1872-1946) was educated at Eton and Trinity College, Cambridge. Only Platt to survive active service in First World War. Gorddinog estate was sold in 1956 on death of his widow.