In the course of Henry VI's reign, Fortescue was appointed one of the governors of Lincoln's Inn three times and served as a Member of Parliament from 1421 to 1437.

He accompanied Queen Margaret and her court while they remained on the Continent[2] between 1463 and 1471, and wrote De Laudibus Legum Angliae for the instruction of young Prince Edward.

[2] He held his office during the remainder of the reign of Henry VI, to whom he was loyal; as a result, he was attainted of treason in the first parliament of Edward IV.

When Henry subsequently fled to Scotland, he is supposed to have appointed Fortescue, who appears to have accompanied him in his flight, Chancellor of England.

During their exile he wrote for the instruction of the young Prince Edward his celebrated work De laudibus legum Angliæ[2] (Commendation of the Laws of England, first published posthumously around 1543),[10] in which he made the first expression of what would later become known as Blackstone's formulation, stating that "one would much rather that twenty guilty persons should escape the punishment of death, than that one innocent person should be condemned, and suffer capitally".

[13] A painted stone effigy of John Fortescue, wearing his scarlet robes of office with collar of ermine, exists within the church, against the north wall of the chancel within the communion rails.

Taken together, Opusculum de natura legis naturæ et de ejus censura in successione regnorum suprema (A Small Work on the Nature of the Law of Nature, and on its Judgment on the Succession to Supreme Office in Kingdoms, c. 1463),[23] De laudibus legum Angliæ (1468–1471), and a work written in English around 1471 which was later published as The Difference between an Absolute and Limited Monarchy (1714)[2] and as The Governance of England (1885), provide the first discussion of the political and conceptual underpinnings of the common law, besides commenting on England's constitutional framework.

[20] De laudibus legum Angliae did not appear in print until about 1543 in the reign of Henry VIII as Prenobilis militis, cognomento Forescu [sic], qui temporibus Henrici sexti floruit, de politica administratione, et legibus ciuilibus florentissimi regni Anglie, commentarius (Commentary on Political Administration and on the Civil Laws of the Most Flourishing Kingdom of England, of the Very Noble Knight, surnamed Forescu [sic], who Flourished during the Reign of Henry VI).

A revised edition of this work, with a historical and biographical introduction, was published in 1885 by Charles Plummer under the title The Governance of England.

Among the surviving works are the pamphlets De titulo Edwardi comitis Marchiæ (The Title of Edward, Earl of March), Of the Title of the House of York, Defensio juris domus Lancastriæ (Defence of the Rights of the House of Lancaster), Replication ageinste the Clayme, and Title of the Duke of Yorke for the Crownes of England and France, as well as the treatise Opusculum de natura legis naturæ et de ejus censura in successione regnorum suprema already mentioned.