Massee had an early interest in natural history, publishing an article on British woodpeckers at the age of 16 and compiling a portfolio of botanical paintings. Through the influence of Richard Spruce, a family relative, he was able to travel on a botanical expedition to Panama and Ecuador, where, despite considerable hardships, he collected orchids and other plants.
On his return, Massee joined the French Foreign Legion, hoping to see combat in the Franco-Prussian War, but, the war being almost over, he was prevailed upon to return home to the farm. He had nonetheless gained a "4th Chasseurs" tattoo on his arm as a result of his brief military adventure.
Back in Yorkshire, Massee developed a particular interest in fungi which he illustrated, his paintings attracting the attention of M.C. Cooke, the first head of mycology at the Royal Botanic Gardens, Kew. Having moved to London, he began a series of ambitious publications on fungi, whilst undertaking public lectures, and also working briefly at the Natural History Museum. When Cooke retired from Kew in 1893, Massee replaced him as Principal Assistant in Cryptogams, a post he retained till he himself retired in 1915. From 1904, Massee's assistant at Kew was A.D. Cotton who worked principally on the algae. In 1910 Elsie Wakefield also assisted Massee with the fungi, taking over his position after 1915.
George Massee helped found the British Mycological Society in 1896 and was elected its first President. He was President of the Quekett Microscopical Club from 1899-1903. During his career, he published over 250 scientific and popular books, papers, and articles on fungi, myxomycetes, plant pathology, and natural history. He was editor of the cryptogamic journal Grevillea for its final two volumes. Massee also described a substantial number of new fungal species, but (despite working in an herbarium) did not always keep type specimens. As a result, all too many of his new species are now relegated to lists of nomina dubia (names of uncertain application). Most of the collections he did retain are now in the mycological herbarium at Kew. Part of Massee's personal herbarium was, however, sold to the New York Botanical Garden in 1907. The fungal genus Masseea was named after him, as were several species, including Acremonium masseei, Ascobolus masseei, Entoloma masseei, and Ocellaria masseeana.