At the École des Chartes he studied palaeography, and was offered a position with the National Archives, but he decided instead to become a lawyer.

At the age of 25 he became director of a warehousing company, and over the next forty years was involved in various major enterprises including the first telephone network in France.

[5] He entered the office of Sébastien Joseph Boulatignier(fr), President of the litigation section in the Council of State.

[13] In 1881, after the first telephony experiments, Lair conceived the great project of organizing a telephone network in France.

[15] Lair was also interested in underwater telegraphy, and wanted to create a network of submarine cable connecting France to her colonies.

The army and navy strongly supported this, but it did not achieve all its goals due to opposition from the Deputy of Le Havre.

[14] His senior management positions in addition to running the Magasins généraux included:[16] Throughout his business career, Lair always set aside time for historical work.

His long preface tried to establish that Dudo was a reliable historical source, which he was not, and the text has been criticized for careless collation and transcription.

[21] While dealing with financiers and competitors, he wrote studies of such people as Louise de La Vallière, Nicolas Fouquet and Cardinal Richelieu.

[13][22] He continued to publish diverse works on aspects of the Middle Ages, the Sun King and the French Revolution.