[3] Like his brother Roland, he collected all kinds of natural history specimens but later restricted himself to plants.

He began medical studied in 1860 and spent some time in 1864 at the University of Edinburgh acting as a clinical assistant to Professor Bennett.

Trimen took part in reforms of the Linnean Society in 1872 and included dates of publication at the backs of the Journal of Botany where he served as editor.

While there, he founded the Museum of Economic Botany and also created subsidiary Gardens at Badulla and Anuradhapura.

His major work at this time was The Flora of Ceylon, which was finished by others after his death.