After university, while working first as a char and then in the Design Centre Bookshop, Parker wrote his first book, The Old Lie: The Great War and the Public-School Ethos, published in 1987. A paperback edition of the book, with a new introduction, was published in 2007. 

 

His second book was a biography of the British writer and editor J. R. Ackerley, published in 1989 under the title Ackerley: A Life of J.R. Ackerley.

 

Parker edited (and wrote much of) The Reader’s Companion to the Twentieth-Century Novel (1995) and The Reader’s Companion to Twentieth-Century Writers (1996).

 

The seeds of Parker’s biography of the writer Christopher Isherwood were sown while he was still at school, where a fellow pupil handed him a copy of Mr Norris Changes Trains, saying ‘I think you’d enjoy this'. The book took twelve years to complete and involved travelling not only to Berlin and Santa Monica, but as far afield as Calcutta. ‘I certainly didn't intend to spend twelve years on Isherwood, and I wouldn't want to do it again,’ Parker has said, ‘but it was the chance to do a full-blown literary biography. When it was authorised, I did feel a certain duty, not to the Isherwood estate but to Isherwood himself, that I had to get in as much of the whole man as I possibly could.’  Isherwood: A Life was published in 2004. 

 

The Last Veteran: Harry Patch and the Legacy of War was published on Armistice Day. The book tells the story of Harry Patch, the longest surviving British veteran of the First World War, and explores the way in which the war has been remembered and commemorated both by those who fought in it and the generations that have followed. 

 

Parker next began to work on the life of A. E. Housman and his most famous book, A Shropshire Lad. Housman Country: Into the Heart of England was published in 2016. The book is a vivid exploration of England and Englishness, in which Parker maps out terrain that is as historical and emotional as it is topographical. It was nominated for the 2017 PEN/Bograd Weld Prize for Biography, was also a New York Times Book Review Editor's Choice, and went on to be chosen as a Book of the Year in the Financial Times, Spectator, Evening Standard and Sunday Times.

 

Parker’s most recent book is A Little Book of Latin for Gardeners, published in 2018. In a series of discursive chapters, he explains how and why plants acquired their botanical names, many of them derived from colour, habit, geography, social and medical history, mythology, folklore, literature, the human body and the animal kingdom. 

 

He was an associate editor of the Oxford Dictionary of National Biography (2004), with responsibility for modern literature, and remains an advisory editor for its regular online updates and the addition of new articles. For the Dictionary he has written articles on J. R. Ackerley, Christopher Isherwood, Harry Patch, Edward Upard, Jean Ross, William Sansom, Simon Raven, Brigid Brophy, Kathleen Hale, Isobel English, Sarah Caudwell, Harry Daley, Esmond and Giles Romilly and the GPO Film Unit. 

  

Parker has also contributed essays to Scribners' British Writers (2002, on L. P. Hartley), Fifty Gay and Lesbian Books Everybody Must Read (2009) and Britten’s Century (2013), and wrote an introduction for the Slightly Foxed edition of Diana Petre’s The Secret Orchard of Roger Ackerley (2016). His edition of G.F. Green’s 1952 novel In the Making was published as a Penguin Modern Classic in 2012. He collaborated on the script and acted as advisor to the producers of the animated feature film My Dog Tulip (2010) based on J.R. Ackerley’s memoir of the same name. 

 

Parker was elected a Fellow of the Royal Society of Literature in 1997, and served as Vice-Chair of the Society’s Council from 2008 to 2014. A former Trustee of the London Library and the PEN Literary Foundation, he was also Chair of the Royal Horticultural Society’s Lindley Library Advisory Committee (2009-2013). He is on the editorial board of ‘A’ Magazine for RIBA Friends of Architecture, for which he is also the books editor and also served on the editorial board the London Library Magazine (2008 - 2019).  He is the chair of the J. R. Ackerley Trust and is one of the judges for the annual PEN/Ackerley Prize for literary autobiography. He also manages Ackerley's official website.