The Combier distillery claims that triple sec was invented some time between 1834 and 1848 by Jean-Baptiste Combier in Saumur, France. However, Combier was more famous for its élixir Combier, which contained orange but also many other flavorings.
According to Cointreau, its orange liqueur was created in 1849.
Triple sec was certainly widely known by 1878; at the Exposition Universelle of 1878 in Paris, several distillers were offering "Curaço [sic] triple sec" as well as "Curaço doux".
^The Combier web site says that Combier's confectionery store opened in 1834, but says only that Triple Sec was invented "à cette époque" as a filling for his candies and chocolates; it also does not mention when Combier's Triple Sec was actually commercialized.
^"Les liquoristes saumurois" in Saumur jadis, anonymous Web publication at  cites Richard Gasnier, Les liquoristes saumurois de 1830 à 1910, mém. de maîtrise, Angers, 2000, B. U. de l'U.C.O., 15 747 ; François Bouyssi et Isabelle Emeriau, "James Combier (1842–1917 ). Essai biographique...", S.L.S.A.S.,, 1992, pp. 46–89 ; Alain Mariez, "Un zeste d'orange, deux doigts d'ambition", L'Anjou, décembre 1995, pp. 70–77 ; Christelle Couvreux, Marie Bardisa, La Distillerie Combier. Saumur, Itinéraires du Patrimoine, 1999.