Johnston was born in 1966 in Guildford, Surrey, England. He began his musical career in earnest in 1988 when he formed the band Gallon Drunk with bassist Mike Delanian. The group's dark, grinding, mutant blues sound earned them a cult following, and after issuing a handful of singles, in 1992 they released their debut album, You, the Night … and the Music, through the British Clawfist label, while it was picked up for American release by Rykodisc. The band's profile in the United States got a massive boost when Morrissey invited them to open his American tour that year, and they were signed to Sire Records for their second album, 1993's From the Heart of Town.
In 1994, Johnston made his solo debut with the album Dora Suarez, a collaboration with former Higsons horn player Terry Edwards, who had played on the From the Heart of Town sessions and would go on to join Gallon Drunk. That same year, Johnston was also hired as a guest musician by Nick Cave the Bad Seeds for their dates on that year's Lollapalooza tour. He later sat in for the recording of Cave's 1996 LP Murder Ballads, and he would go on to become a full member of the Bad Seeds, touring with the group and appearing on the albums Abattoir Blues/The Lyre of Orpheus (2004) and Dig, Lazarus, Dig!!! (2008).
Despite his many other projects, Johnston continued to work with Gallon Drunk, issuing four albums between 1996 and 2007 (In the Long Still Night, Black Milk, Fire Music, and The Rotten Mile). In 2009, he teamed up with electronic musician Philippe Petit for the album Fiends with a Face. Johnston was invited to sit in with the iconic German experimental band Faust for their 2011 album Something Dirty, and the following year Gallon Drunk reconvened to issue a studio album, The Road Gets Darker From Here. Guitarist Cypress Grove and vocalist Lydia Lunch brought in Johnston to play on their first collaborative project, 2014's A Fistful of Desert Blues, and he would join them in the studio again for their 2017 effort Under the Covers. He also recorded and toured with Lunch as part of her group Big Sexy Noise.
PJ Harvey tapped Johnston to play in her studio band for 2016's The Hope Six Demolition Project, as well as touring behind the release, and he would later work with her for the recording of her score for the 2019 stage musical adaptation of the classic film All About Eve. Also in 2016, Johnston issued The Starless Room, his first solo album not created in collaboration with another artist. During his downtime from music, Johnston often focuses on visual art, having earned positive notices from art critics for his paintings. Significantly, in 2021 he worked with a visual artist who also makes music, photographer Steve Gullick, to create the album We Travel Time. ~ Mark Deming, Rovi