Born in Rome, he entered the Jesuitnovitiate in 1828, and later served as instructor in grammar, for which his philological bent particularly fitted him, at Rome, Modena and Reggio Emilia. While studying at the Collegio Romano in 1839, a letter of Joseph Rosati, Bishop of St. Louis, voicing the appeal of the Flatheads for missionary priests, was read out in the refectory, during the meal, and Mengarini felt moved to volunteer for the work. Ordained priest in March 1840, he sailed with Father Cotling, another volunteer, from Livorno on 23 July, and after a nine weeks' voyage landed at Philadelphia. From Baltimore the missionaries found their way to the University of Georgetown, District of Columbia, and a little later to St. Louis, where it was decided Cotling should remain. Mengarini was chosen for the mission of the upper Missouri, partly on account of his voice and knowledge of music – valued in Indian mission work. On 24 April 1841, Father de Smet, Mengarini, and Nicolas Point, with the lay brothers Specht, Huett, and Classens, and nine other companions, began the long journey by river and overland trail to Fort Hall, Idaho, then a trading post, where they arrived on the feast of the Assumption and found a party of Flatheads waiting to conduct them to their final destination. It was nearly a month later when they arrived at the chosen site on St. Mary's River, Montana, in the Flathead country, and began the foundations of the log mission. The missionaries worked at cutting the frozen earth with axes. The church and house were of logs plastered between with clay, and were thatched with weeds, the rooms being partitioned with curtains of deerskin and thin scraped deerskin being used in lieu of glass for the windows. The winter cold was so intense that the buffalo skin robes in which they wrapped themselves at night were frozen stiff and had to be thawed out each morning. The missionaries began the study of the language, translating into it simple prayers and hymns. Mengarini composed a Salish grammar which is still the standard for the cognate dialects. He taught the children to sing in Salish hymns of his own composition, and even trained an Indian band for service on feast days. The work progressed until 1849, when in consequence of the inroads of the Blackfeet and the defection and relapse of a large part of the Flathead tribe under a rival claimant for the chieftainship it was decided to close the mission. Mengarini was summoned to join Father Accolti, the superior of the north-western Jesuit Missions, in Oregon. About a year later, on request of Archbishop Alemany of San Francisco for Jesuit workers, he was sent to aid in establishing at Santa Clara the Californian mission which was the nucleus of the present college. In the meantime the Flatheads had sent to Oregon to ask for his return. They were told this was impossible as he was assigned to another station, but on their urgent desire, the Flathead mission was re-established at St. Ignatius in 1851. Mengarini was stationed at Santa Clara for the rest of his life, acting for thirty years as treasurer or vice-president, until a stroke of apoplexy and failing sight caused his retirement from active duties. A third stroke of apoplexy ended his life in his seventy-sixth year.
Mengarini's principal contribution to philology is his Selish or Flathead Grammar: Grammatica linguæ Selicæ – published by the Cromoisy Press from the third manuscript copy, the first two, laboriously written out by him, having been lost. Originally intended solely for the use of the missionaries, it was written in Latin, and he himself always said the first draft was the most correct. He also furnished vocabularies of the cognate Salishan languages – of Shw oyelpi, S'chitzui and Salish proper in John Wesley Powell's Contributions to North American Ethnology, I, and of the Santa Clara dialect of California in Stephen Powers's Tribes of California, volume III of the same series, published in the same year. He contributed some linguistic notes in the Journal of the Anthropological Institute of New York, I. His personal memoir, The Rocky Mountains, published in the Woodstock Letters for 1888, was dictated a few months before his death.
