Andrew's first mission to the East was when he was asked by the French king Louis IX to go and fetch the Crown of Thorns which had been sold to him by the Latin king of Constantinople Baldwin II in 1238, who was anxious to obtain support for his tottering empire. Andrew was accompanied on this mission by brother Jacques.
In reply to this the French sovereign dispatched Andrew as his ambassador to Güyük Khan; with Longjumeau went his brother William (also a Dominican) and several others — John Goderiche, John of Carcassonne, Herbert "Le Sommelier," Gerbert of Sens, Robert (a clerk), a certain William, and an unnamed clerk of Poissy.
The party set out on 27 January 1249, with letters from King Louis and the papal legate, and rich presents, including a chapel-tent, lined with scarlet cloth and embroidered with sacred pictures. From Cyprus they went to the port of Antioch in Syria, and thence traveled for a year to the Khan's court, going ten leagues (55.56 kilometers) per day. Their route led them through Persia, along the southern and eastern shores of the Caspian Sea, and certainly through Talas, north-east of Tashkent.
On arrival at the supreme Mongol court — either that on the Imyl river (near Lake Alakol and the present Russo-Chinese frontier in the Altay), or more probably at or near Karakorum itself, south-west of Lake Baikal — Andrew found Güyük Khan dead, poisoned, as the envoy supposed, by Batu Khan's agents. The regent-mother Oghul Qaimish (the "Camus" of William of Rubruck) seems to have received and dismissed him with presents and a letter for Louis IX, the latter a fine specimen of Mongol insolence. But it is certain that before the friar had quit "Tartary" Möngke, Güyük's successor, had been elected.
Andrew's report to his sovereign, whom he rejoined in 1251 at Caesarea in the Palestine, appears to have been a mixture of history and fable; the latter affects his narrative of the Mongols' rise to greatness, and the struggles of their leader Genghis Khan with Prester John; it is still more evident in the position assigned to the Mongols' homeland, close to the prison of Gog and Magog. On the other hand, the envoy's account of Mongol customs is fairly accurate, and his statements about Mongol Christianity and its prosperity, though perhaps exaggerated (e.g. as to the 800 chapels on wheels in the nomadic host), are based on fact.
Mounds of bones marked his road, witnesses of devastations which other historians record in detail. He found Christian prisoners from Germany in the heart of "Tartary" (at Talas), and was compelled to observe the ceremony of passing between two fires, as a bringer of gifts to a dead Khan, gifts which were of course treated by the Mongols as evidence of submission. This insulting behaviour, and the language of the letter with which Andrew reappeared, marked the mission a failure: King Louis, says Joinville, "se repenti fort" ("felt very sorry").
We only know of Andrew through references in other writers: see especially William of Rubruck's in Recueil de voyages, iv. (Paris, 1839), pp. 261, 265, 279, 296, 310, 353, 363, 370; Joinville, ed. Francisque Michel (1858, etc.), pp. 142, etc.; Jean Pierre Sarrasin, in same vol., pp. 254–235; William of Nangis in Recueil des historiens des Gaules, xx. 359–367; Rémusat, Mémoires sur les relations politiques des princes chrétiens… avec les… Mongols (1822, etc.), p. 52.