The name, Trengwainton, means "The settlement of spring" in the Cornish language. It probably refers to the fact that plants such as rhododendrons may start blooming in November in this garden. This area gets around 50 inches of rain per year so there is plenty of moisture for a wide variety of plants. Many tropical and subtropical plants will survive here that will not grow in other parts of England. From the top of the garden, you can see St. Michael's Mount  and the Lizard Peninsula.

There has been a dwelling at Trengwaiton since around the 16th century. In 1814, it was purchased by a rich sugar planter from Jamaica who started planting trees on the estate. In the early 20th century, G.H. Johnstone of Trewithen and Lawrence Johnston of Hidcote Manor offered plants from an expedition they had sponsored to Asia. Many of the rhododendrons on the property came from seeds collected on that expedition.

The gardens started opening to the public in the 1930's and, like most large British gardens, fell into neglect during World War II. Areas were opened up to vegetable production to support the war efforts.

In 1961, the gardens were turned over to the National Trust.