Born in Maryland to parents who worked as language analysts for the National Security Agency, Callahan spent his childhood living in his birthplace and the North Riding of Yorkshire, England. By the late '80s, he was making music as Smog, recording painfully intimate songs that ping-ponged wildly through a scrapbook of childhood recollections, failed relationships, bizarre fetishes, and dashed hopes on a four-track recorder. Smog debuted in 1988 with the spare, primitive Macrame Gunplay, a cassette-only release issued on Callahan's own Disaster label. After signing to Drag City in 1991, Callahan added more melody to his music while remaining true to Smog's trademark bare-bones atmosphere on albums including 1991's Forgotten Foundation, 1993's Julius Caesar, and 1995's Wild Love. Two years later, he took another leap forward with 1997's Red Apple Falls, which added folk and country influences that complemented his increasingly thoughtful songwriting. Callahan's music became more meditative on 2000's Dongs of Sevotion and the following year's hypnotic Rain on Lens, for which he changed his project's name to (Smog). His final album under that name, the literary, laid-back A River Ain't Too Much to Love, appeared in 2005. In 2007, Callahan began releasing his music and other projects under his own name. That year's Diamond Dancer EP and full-length Woke on a Whaleheart both mixed the reflective, largely acoustic sound of later Smog albums with gospel, soul, and pop elements, and boasted arrangements by Royal Trux's Neil Hagerty. On 2009's Sometimes I Wish We Were an Eagle, Callahan worked with engineer John Congleton and Brian Beattie, who provided string and brass arrangements. The live album Rough Travel for a Rare Thing, which captured a show in Melbourne, Australia, arrived in March 2010; in July, Callahan issued his 79-page "epistolary novelette" Letters to Emma Bowlcut, comprised of 62 letters from a nameless protagonist to a woman he saw at a party. He reunited with Congleton and Beattie for 2011's Apocalypse, a more up-tempo collection of seven country- and blues-inspired rock tunes that recalled some of his edgier work with Smog. The following year, a documentary of the Apocalypse tour directed by photographer/filmmaker Hanly Banks was released. In January 2013, The Life and Times of William Callahan, a book of photographs by Canadian photographer Chris Taylor, was published; that September, Callahan issued Dream River, an album of gentle songs to listen to at the end of the day. The following year, Have Fun With God offered haunted dub versions of Dream River's songs. Also in 2014, Callahan published I Drive a Valence, a collection of his drawings and lyrics from his entire career, and he married Banks.
During the second half of the 2010s, Callahan took a break from releasing music to focus on family life. He and Banks had a son in 2015, and his mother died in 2018. During this time, Callahan continued to write songs and tour; late in 2018, the concert album Live at Third Man appeared. He returned in June 2019 with Shepherd in a Sheepskin Vest, a flowing, confessional set of songs recorded with Beattie and guitarist Matt Kinsey. ~ Heather Phares, Rovi