He is Professor Emeritus of Integrative Biology and was Curator of Amphibians and Reptiles at Oregon State University, Corvallis until his retirement.

[5][7] In 1971, he moved back to Berkeley to begin a Miller Postdoctoral Fellowship with David Wake, launching a new research program on the behavioral ecology of garter snakes.

During this period, he was especially influenced by his colleagues Michael Wade and Russell Lande as his interests moved in the direction of evolutionary quantitative genetics.

[5] After his retirement he donated approximately 50,000 specimen snake and amphibian collection assembled with Lynne Houck to the University of Michigan Museum of Zoology.

[8] Arnold's work has been mainly focused in the field of evolutionary quantitative genetics, specifically on evolution of phenotypic traits (body size, morphology, behavior, whole organismal performance) that are affected by many genes.