Marion read English at Oxford and then spent a year abroad before reading for an MA in Medieval Studies at York. She returned to Oxford to study for a doctorate, focusing on Chaucer, which she gained in 2002. She was a Fellow by Examination (Prize Fellow) at Magdalen College, Oxford, and also taught at King's College London before returning to Oxford in 2007 to take up a fellowship at Jesus. In 2006, she was awarded a research fellowship by the Leverhulme Trust and in 2011 she was awarded research funding from the Wellcome Trust.
Her research interests lie in late medieval secular literature and history, especially Chaucer, the Gawain-poet, and political texts. Her first book - Chaucerian Conflict - came out with Oxford University Press in 2007 and she recently edited A Handbook of Middle English Studies for Wiley-Blackwell, as part of their Critical Theory Handbook series. She has published many articles on late medieval literature and history. She is also interested in literature and medicine, and in the relationship between space and textual production. Her current major project is a substantial biography of Chaucer for Princeton University Press.