Sir William Thomas was a Caernarfonshire landowner and public official. He inherited the Thomas estates, including Aber, and built the Jacobean house at Pen-y-bryn at the manor of Aber around 1600. He obtained a further Crown grant or confirmation of Aber and Cemaes in 1609.

His settlement disinherited his eldest son John and placed most of the estate on his second son William.

Research sources:
- Dictionary of Welsh Biography, 'Thomas family, of Coed Helen (or Alun) and Aber, Caernarfonshire'; University of Wales Bangor Archives, BMSSXII/21171, item 21281, Grant by letters patent to Sir William Thomas of the manor of Aber, temp. James I. — https://biography.wales/article/s-THOM-COE-1500.html