Tunnicliffe worked in several media, including watercolor painting, etching and aquatint, wood engraving, woodcut, scraperboard (sometimes called scratchboard), and oil painting. Much of Tunnicliffe's work depicted birds in their natural settings and other naturalistic scenes. He illustrated Henry Williamson's Tarka the Otter. His work was also used to illustrate Brooke Bond tea cards and as a result was seen by millions of young people in the United Kingdom during the 1950s and 1960s. He also illustrated a number of books, including the Ladybird books. His work was characterised by its precision and accuracy, but also by the way in which he was able to portray birds as they were seen in nature rather than as stiff scientific studies. From March 1953, he painted many of the cover illustrations for the Royal Society for the Protection of Birds's (RSPB) magazine Bird Notes, and several for the later Birds magazines. Two of the originals are on long-term loan to the gallery at Oriel Ynys Môn, but in 1995 the RSPB sold 114 at a Sotheby's auction, raising £210,000; the most expensive being a picture of a partridge, which sold for £6,440. At his death, much of his personal collection of work was bequeathed to Anglesey council on the condition that it was housed together and made available for public viewing. This body of work can now be seen at Oriel Ynys Môn (The Anglesey Gallery) near Llangefni. His work is still celebrated with the Charles and Winifred Tunnicliffe Memorial Art Competition, which is held annually at Hollinhey Primary School, Sutton, which itself is built on land which was formally part of the farm he lived on as a boy.