Willis was an early member of the British Archaeological Association, and gave his paper on Canterbury Cathedral at their first meeting in 1844. Soon after this, the Association split, leading to the formation of the Archaeological Institute (later Royal). Willis transferred to the new Institute, giving his paper on Winchester Cathedral at their first meeting the following year. In 1849 a Royal Commission was set up to investigate the use of iron in railway structures. Willis was one of the commissioners, and carried out experiments to determine the effects of moving loads on iron structures. He was one of the jurors in the Great Exhibition of 1851, and his lecture "On machines and tools for working in metal, wood and other materials" was published in the following year. In 1855 he served as vice president of the Paris Exposition, and in 1862 received the Royal Gold Medal in architecture.