[2] From 2003 to 2009, he published a sequence of biographies of medieval political leaders: first Roger Mortimer, 1st Earl of March, then Edward III, and Henry IV, in addition to 1415, a year in the life of Henry V.[2] Mortimer's best-known book is The Time Traveller's Guide to Medieval England, first published in the United Kingdom in 2008.
[2] He is also well known for pioneering, in his first two books and an article in The English Historical Review, the argument (based on evidence such as the Fieschi Letter) that Edward II did not die in Berkeley Castle in 1327.
In this essay he demonstrated that ill and injured people close to death shifted their hopes of physical salvation from an exclusively religious source of healing power (God, or Jesus Christ) to a predominantly human one (physicians and surgeons) over the period 1615–1670, and argued that this shift of outlook was among the most profound changes western society has ever experienced.
[citation needed] In 2011, Mortimer entered the genre of historical fiction, publishing the first book from his Elizabethan era Clarenceux Trilogy using the pen name of James Forrester.
[citation needed] In a 2010 essay by Mortimer, he criticised Wikipedia's spurning of primary sources, and its apparent inability to publish the latest research done by experts in a field, citing his involvement with editing the birth date of Henry V as an example.
In this piece, he opined that "the structure of Wikipedia promotes hearsay, prejudice, supposition and superficiality on an equal footing with genuine information and understanding", and that Wales created the resource for his own political ends.