The house was apparently built at the end of the 16th century by Ellis ap Cadwaladr (d. 1597),[1] a member of the Welsh Ellis family, which claimed to trace its lineage back to Gollwyn ap Tangno, founder of one of the Fifteen Tribes of Wales.

[4] In c. 1946, J. Egbert Griffiths of Porthmadog carried out several minor alterations, adding some extra windows and rearranging the internal room structure.

[2] John Ystumllyn (d. 1786), who took his name from the household, was employed by the Wynn family at this estate as a gardener and survived as the first well-recorded black person of North Wales.

He was of uncertain origins, and was possibly kidnapped from Africa by the Wynn family, but lived out a happy life in Ystumllyn, eventually running away with and marrying a local woman.

[5] Ystumllyn is a rubble-built house of two stories, composed of two blocks arranged into a T-shaped plan.