Armstrong was educated at local schools and then read philosophy and English at Sunderland Polytechnic (now the University of Sunderland). While at polytechnic, he was converted from Roman Catholicism to Evangelical Protestantism, but more recently has described himself as an "Anglo-Catholic agnostic". He trained as a teacher but then turned to psychiatric nursing, and now works as a cognitive behavioural psychotherapist. He lives in Stocksfield, Northumberland.

Armstrong began to publish his poetry in the late 1970s, contributing to magazines and to Ten North-East Poets, The Firebox. Poetry in Britain and Ireland after 1945, and Last Words: New Poetry for the New Century, and other collections. He won an Eric Gregory Award from the Society of Authors in 1984. His first volume of poetry, Risings, appeared in 1988 and has been followed by three others.

Armstrong is one of the editors of the Newcastle-based magazine Other Poetry, which was revived in 1995 and appears three times a year. He is also a member of the Northern Poets' Workshop. His work is marked by joint preoccupations with religion and the landscape---of his native North-East, but also of the Hebrides and of a notional American drawn from road movies and cigarette advertising. His verse has been influenced more recently by his work as a cognitive therapist.

Source: British Library Integrated Catalogue:

• Risings (Petersfield: Enitharmon, 1988. ISBN 1870612000)
• The Red-Funnelled Boat (London: Picador, 1998. ISBN 0330369148)
• The Capital of Nowhere (London: Picador, 2003. ISBN 0330412671)
• Madame Noire (and other figures at the edge of an imagined war) (Nottingham: Shoestring, 2008. ISBN 1904886744)

Five poems by Peter Armstrong (born 1939) appeared in 1969 in the anthology Poetry of the Underground in Britain, edited by Michael Horovitz and dedicated to Allen Ginsburg. These were taken from the booklet 28 Poems (Bristol: View Publications, 1966).

Otterburn 1388. Bloody Border Conflict by Peter Armstrong (Illustrated by Stephen Walsh. Oxford: Osprey, 2006. ISBN 1841769800) tells the story of a medieval battle between England and Scotland.