Llewellyn was born in Cirencester and began studying art in South Kensington under Poynter in 1879. In 1887 he made his first visit to Cornwall. The following year he spent some time in Polperro where his highly acclaimed work Twixt Night and Day - a Cornish Harbour was painted and shown at the Grosvenor Gallery and at Manchester Art Gallery. In 1927 it was presented to Dunedin Public Art Gallery in New Zealand.

After continuing to visit Cornwall regularly, in 1893 Llewellyn married the miniature portrait painter Marion Meates, settled in London and became a highly successful portraitist. He was knighted in 1918, made a Royal Academician in 1920 and served as its president between 1928 and 1938.

A portrait of William Coles Pendarves, JP (b.1841), Chairman of the Council (1907-1919) was painted by this artist in 1918, and is part of the permanent collection of Cornwall Council.