After graduating from secondary school he entered the order of Jesuits in Vienna in 1735. He studied in Vienna and Graz, (Austria) where he lectured in mathematics. By the year 1751, he was in Lisbon, Portugal where he obtained the title of royal mathematician and astronomer. With those credentials, he became a member of the expedition that worked on the rearrangement of the frontiers among colonies, (Portugal and Spain), in South America.
In 1753, he sailed for Brazil into the very mouth of the Amazon River. Based on his surveys, Lorenz Kaulen made in 1753 a map of Maragnon district titled Mappa Viceprovinciae Societatis Iesu Maragnonii anno MDCCLIII concinnata. Its original is in Biblioteca de Evora in Portugal. Another map, Mapa da Ilha do Maranhão, made in 1757 again based upon Szentmartony's surveys, is also kept here. From 1754 - 1756 Szentmartony took part in expeditions to the Amazon and the Rio Negro. Upon the data obtained from his systematical astronomical surveys, in 1755 engineers Schwebel and Sturm made a regional map titled Mappa Geographico dos Rios. It was a first-rate cartographic representation of the riverbed containing data relating to islets, tributaries and settlements. Based on that map Miguel Vieira Perreira made a copy in 1862 which was first published in 1963 in Rio de Janeiro.
He complained vigorously about the inhumane treatment of the native people by the colonizers, so his expedition came under scrutiny and failed. Szentmartony remained as a missionary in the settlement of Ibyrajuba near Pará. In 1760 he was deported with other persecuted Jesuits, put in prison and released in 1777 upon the intervention of the Empress Maria Theresa. He returned to Croatia. Before that, in 1773 his religious order was cancelled. He was priest in Belica and died in Čakovec in 1793.
Ignacije Szentmartony younger, February 17, 1743, was educated in theology in Varaždin and Graz, lecturer in Varaždin and Zagreb where he wrote the first Croatian kajkavian grammar in the German language, Einleitung zur kroatischen Sprachlehre für Teutschen. It was published in Varaždin in 1783.