Kocan was born Peter Raymond Douglas in Newcastle, New South Wales and raised in Melbourne. His father, an engineer, was killed in a car accident three months before his birth. Kocan's mother then moved to Melbourne and remarried, but the marriage failed so Peter, his mother and younger brother moved to Sydney. Escaping a childhood of loneliness, disadvantage and family violence, Kocan left school at fourteen to work in country New South Wales as a labourer and station-hand, before returning to Sydney, where he gained work as a factory-hand in a dye factory.

On the evening of 21 June 1966, while campaigning for the 1966 federal election, Calwell addressed an anti-conscription rally at Mosman Town Hall in Sydney. After Calwell left the meeting, just as his car was about to drive off, Kocan approached the passenger side of the vehicle, aimed a sawn-off rifle at Calwell's head and fired at point-blank range. The closed window deflected the bullet, which lodged harmlessly in Calwell's coat lapel, and he sustained only minor facial injuries from broken glass. Calwell later visited Kocan in the mental hospital where he was confined for ten years, and forgave him for the incident.

Kocan was tried and found guilty of attempted murder. He was sentenced to life imprisonment and was detained first at Long Bay Correctional Centre in Sydney; in late December 1966, Kocan was transferred from jail to Ward 6 for the Criminally Insane in Morisset Psychiatric Hospital on the NSW Central Coast.

During his years in prison and hospital, Kocan immersed himself in history, literature and poetry, after a chance remark by another inmate led him to discover the writing of Rupert Brooke. Between 1967 and 1969, the poet Michael Dransfield corresponded and exchanged poems with him. These letters, which comprise drafts of poems by Dransfield, quotes of poems by other poets and recommendations for books Kocan should read, are now held in the collection of the Academy Library of the University of New South Wales.

Kocan began to write poetry in 1967. Two selected works of poetry, Ceremonies for the lost (1974) and The other side of the fence (1975) were published while he was at Morisset. He was released on license from Morisset in August 1976, and began to rebuild his life by writing about his experiences. Two autobiographical novellas, The Treatment and The Cure, told of his harrowing life in the asylum. The Cure won the 1983 NSW Premier's Literary Award for Fiction. He has more recently published poetry, including the volumes "Standing with Friends" and "Fighting in the Shade" and the joint collection "Primary Loyalties" which move away from personal experiences towards themes of universal concern. He has also written a science-fiction/fantasy novel, "Flies of a Summer," set in a savage future world where memory of the past has been erased.

Kocan lived for many years on the Central Coast of New South Wales, teaching, acting, and writing drama, poetry, and fiction. He gained public recognition for his work and received regular support from the Literary Arts Board of the Australia Council and has won various literary prizes. He graduated from the University of Newcastle in 1998 with a Bachelor of Arts (Honours), and recently obtained a Masters degree. He moved to Brisbane in 2003. His most recent novel, Fresh Fields, is a fictionalised account of his youth.