William Arthur Reynolds was born April 20, 1930 in Havre, Montana, into a family of medical professionals. Reynolds' grandparents, William P. Reynolds and Winifred Reynolds, worked as doctors in Nova Scotia, Canada: Lane, South Dakota: and later Aldrich and Stevensville, Montana. His father, F. Gordon Reynolds, worked as a dentist in Missoula, Montana, while his mother Nettie Porter-Reynolds (of the Buck-Porter family), trained as a teacher and later helped in Gordon Reynolds' dentistry office. Nettie's father, William A. Reynolds' maternal grandfather, Arthur T. Porter and his brother ran the Buck and Porter Brothers Grocery Store in Stevensville, Montana up until 1922.

Living in Missoula, Montana during his childhood, William A. Reynolds attended Paxson Missoula elementary school, Missoula County High School, and Montana State University in Missoula (now called the University of Montana). While at Montana State, William ran varsity track and was student body president during his senior year. Upon receiving his pre-medical degree at Montana State University, he went to medical school at the Washington University School of Medicine in Saint Louis, Missouri. He met his wife, Joanne Flanagan, through mutual friends while in school there. William Reynolds completed his medical internship at the University of Minnesota Hospitals, and completed his internal medicine fellowship and Masters of Surgery at Mayo Clinic in Rochester, Minnesota. Between completing his internship and completing his residency, Reynolds was called to active military duty and served as medical personnel on the Nouasseur Air Force Base in Morocco. Upon completion of his military service, Reynolds returned to the Mayo Clinic to complete his residency.

Upon completing his medical education and military service, William moved his family back to Missoula where he worked at the Western Montana Clinic, practicing internal medicine and endocrinology. William Reynolds has served as president of St. Patrick Hospital Medical Staff (1974 to 1975 and 1987 to 1988), Western Montana Clinic (1975 to 1979), and the Montana Society of Internal Medicine (1976). Reynolds also served as a professor of Medicine at the University of Washington School of Medicine, Missoula Unit from 1974 to 1994. In 1997, he was elected president of the American College of Physicians after serving as a Governor and Regent for the Montana Chapter. Reynolds retired in 2001. In May 2019, Reynolds received an Honorary Doctorate of Humane Letters from the University of Montana. Reynolds died August 5, 2021 in Missoula, Montana.