Mullen's paintings are a contemporary take on southern wildlife and culture, heavily influenced by antique field guides and early scientific illustration. Using a realist painting technique, they draw attention to anatomy and juxtaposition of the environment both organic and man-made. There is an underlying tension of balance through transition. The artist carefully contextualizes the unique natural history of the south to convey underlying themes of nature, culture, and place.

Since Mullen's debut solo exhibition, held in the gallery room of the Louisiana state archives, his work has been exhibited in gallery spaces in Los Angeles, Miami, San Fransisco and internationally. His paintings are part of various private collections across the world.