Most of Johnson's poetry is political, dealing mainly with the experiences of being an African-Caribbean in Britain, "Writing was a political act and poetry was a cultural weapon...", he told an interviewer in 2008. However, he has also written about other issues, such as British foreign policy or the death of anti-racist marcher Blair Peach. His most celebrated poems were written during the government of Prime Minister Margaret Thatcher. The poems contain graphic accounts of the alleged racist police brutality occurring at the time (cf. Sonny's Lettah). Johnson's poetry makes clever use of the unstandardised transcription of Jamaican Patois.

Johnson's poems first appeared in the journal Race Today, which published his first collection of poetry, Voices of the Living and the Dead, in 1974. Dread Beat An' Blood, his second collection, was published in 1975 by Bogle-L'Ouverture.

A collection of his poems has been published as Mi Revalueshanary Fren by Penguin Modern Classics. Johnson is one of only three poets to be published by Penguin Modern Classics while still alive.

Johnson's best-known albums include his debut Dread Beat an' Blood, Forces of Victory, Bass Culture and Making History. Across these albums are spread classics of the dub poetry school of performance – and, indeed, of reggae itself – such as "Dread Beat An' Blood", "Sonny's Lettah", "Inglan Is A Bitch", "Independent Intavenshan" and "All Wi Doin Is Defendin".His poem Di Great Insohreckshan is his response to the 1981 Brixton riots. The work was the subject of a BBC Radio 4 program in 2007.

Johnson's work, allied to the Jamaican "toasting" tradition, is regarded as an essential precursor of rap.

Johnson's record label LKJ Records is home to other reggae artists, some of whom made up The Dub Band, with whom Johnson mostly recorded, and other Dub Poets, such as Jean "Binta" Breeze.

Of late, Johnson has only performed live on an intermittent basis, perhaps as a result of modern reggae's shift towards the more spontaneous and rapid-fire performers of ragga or dancehall.