He was born in Modesto, California. In 1908, he obtained his medical degree from Johns Hopkins University, eventually becoming its associate professor of anatomy. Evans moved back to California in 1915 and was made professor of anatomy at the University of California, in Berkeley, California, and held that position until his death.
His medical research at Berkeley addressed problems relating to human nutrition, endocrinology, embryology, and histology. In 1918, his research into the number of human chromosomes led him to believe the number to be 48, when most people assumed the number to be much higher. It was only later discovered that the correct figure was 46. Evans had much greater success however with hormones extracted from the anterior lobe of the pituitary gland. He isolated Human Growth Hormone, which was essential for human growth and development. In 1922 along with Katharine Scott Bishop, during feeding experiments on rats, he co-discovered Vitamin E which is needed for human reproduction. Evans became director of the Institute of Experimental Biology at Berkeley, in 1931. He continued his research on vitamine E and was finally able to isolate the pure compound from wheat germs in 1937. He also determined the formula C29H50O2. He was also instrumental in developing reproductive systems research with Miriam Sipson and C.H. Li, by studying the oestrus cycle of rats. Evans is also credited with developing Evans blue, a method which determines blood volume in humans and animals.
He died in Berkeley, California, aged 88.