Gregorio Mengarini, born the 21 July 1811 and dead the 23 September 1886, was an Italian Jesuit priest and missionary and linguist. He worked as a pioneer missionary in the north-west of the United States to the Flathead Nation, and became the philologist of their languages.
Born in Rome, he entered the Jesuit novitiate 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 (15 August) 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.