His first position was as a councillor at the Châtelet and then he was treasurer of the généralité of Amiens. A généralité was a French administrative unit, each under a "collector general" (giving the unit its name) who was a direct agent of the king with wide police, justice, and financial powers. The position of treasurer general was a later addition to the généralités which came about after 1577. This position was an ideal one for Mydorge for it was a high ranking important position, and therefore a worthy one for a member of a leading French family, yet it had light duties which meant that Mydorge could devote a large amount of his time to the subject he loved, namely mathematics. In 1613 he married Mme de la Haye, the sister of the French ambassador in Constantinople.
Mydorge studied geometry and physics. He published books on optics and conic sections, for example De sectionibus conicis, libri quattuor
A further portion of the work, in manuscript, is lost. It seems that two English friends of the Mydorge family, William Cavendish, Duke of Newcastle, and Thomas Wriothesley, Earl of Southampton, took it to England, where apparently it disappeared.Mydorge's work simplifies many of Apollonius's proofs. However, it contains a new powerful idea, namely that of deforming figures. For example he shows how to deform a circle into an ellipse and proves other results on deforming conic sections. The technique was taken up by La Hire and Newton, then later by Poncelet and Chasles.
He was interested in mathematical recreations and edited Récréations Mathématiques
Mydorge left an unpublished manuscript Traité de géométrie
Upon the given side ab, with midpoint g, construct the equilateral triangle abc and extend the altitude gc to point d such that cd is one-sixth ab. Then c is the centre of the circle circumscribing the desired heptagon, and the construction is easily completed.However Mydorge does not state that the construction is only an approximation. In  Busard examines the magnitude of the error in this and other constructions of Mydore in Traité de géométrie
It was not only mathematical problems which interested Mydorge. He also worked on light and refraction in particular. His interest in optics also fitted in with an interest in making astronomical observations. He was a close friend of Descartes and made a large number of optical instruments for him, the two shared a strong interest in explaining vision, and the instruments and lenses were to help develop theories.
One of Mydorge's most famous results was an extremely accurate measurement of the latitude of Paris. He was also interested in methods of determining longitude and was appointed to a committee to determine the whether Jean-Baptiste Morin's methods for determining longitude from the Moon's motion was practical. Hérigone and Étienne Pascal served with him on this committee.
Speziali writes in :-
A friend of Descartes and an eminent geometer, Mydorge was also well versed in optics. He possessed a lively curiosity and was open to all the new ideas of his age. Like Fermat, he belonged to that elite group of seventeenth-century scientists who pursued science as amateurs but nevertheless made contributions of the greatest importance to one or more fields of knowledge.
Article by: J J O'Connor and E F Robertson