William Grant "Will" Bagley was born on May 27, 1950 in Salt Lake City, Utah to Cassandra Margene Bailey and Lawrence Miles Bagley. When Will was a child, the Bagley family moved to Oceanside, California, where Lawrence Bagley was a city planner and then mayor. Bagley was raised in the faith of the Church of Jesus Christ of Latter-Day Saints, but as he grew to adulthood he questioned its theology. In 2002, he told the Ex-Mormon Conference that he was a "heritage Mormon." Will Bagley graduated from Oceanside High in 1967. He studied at Brigham Young University, transferring to the University of California Santa Cruz after a year. There, he studied with Page Stegner and John Dizikes. He graduated with a BA in History in 1971.

>Bagley had a lifelong interest in the outdoors and in music. In college, Bagley built and piloted a raft down the Mississippi River as an independent study, the first of two such trips that he chronicled in his book (2019). For several years after returning to Utah, Bagley pursued a career in music, recording an LP called "The Legend of Jesse James." In 1978, Bagley married Janis Johnson and after the birth of his daughter in 1980, Bagley left the music industry and began working as a technical writer in the computer industry. Bagley and Janis Johnson divorced in 2001 and in 2003 Bagley remarried to Laura Bayer.

In 1990, Bagley began writing history, and his first book, , was published in 1991. Bagley wrote many works of history for all ages, including the 1996 children's book , a collaboration with his brother, artist Pat Bagley, and the column "History Matters" in the Salt Lake Tribune, which was published between 2000 and 2004. Bagley was also a prolific editor, serving as editor of the Kingdom in the West series for the publisher Robert A. Clark from 1997 until 2019; serving on the executive committee of the ; and editing two volumes for the Utah, Mormons, and the West series sponsored by the University of Utah and the Tanner Trust. He also performed historical research for the National Park Service and the Oregon California Trails Association. Bagley also communicated about history in other mediums, notably appearing frequently on Doug Gabrizio's program at KUER and serving as a consultant for the 2007 PBS documentary .

Around 1990, Bagley began researching the Mountain Meadows Massacre, publishing his most well-known word, , in 2002. The volume was well received by historians but received criticism from groups affiliated with the Church of Jesus Christ of Latter-Day Saints. The study received numerous awards, including the Spur Award from the Western Writers of America and the Caughey Book Prize from the Western History Association.

Bagley served on a number of organizations throughout his career, including: Western Writers of America, Utah Westerners, Friends of the Marriott Library, Oregon California Trails Association, Utah Rivers Council, Friends of the Great Salt Lake, Western History Association, Mormon History Association, the Utah State Historical Society, and others. Throughout his career, Bagley published more than 25 books and hundreds of articles related to the history of Utah and the West.

Will Bagley died on September 28, 2021. (Biographical note adapted from obituary.)