mcnary was born on january 31, 1867, on a farm in marion county, oregon, to hugh linza mcnary and margaret claggett mcnary. the mcnary family farm was north of salem, where mcnary's parents would raise ten children. mcnary's grandfather was james mcnary who immigrated to oregon country from kentucky in 1845, while his maternal grandfather immigrated from missouri in 1852. his father hugh was a former brickyard operator and school teacher. his mother died in 1878; his father died in 1883. mcnary was educated in the local public schools in marion county before attending willamette university. john received his college education at the university of oregon in eugene. in 1890, he was elected as the recorder for marion county. after college he read law under the guidance of george h. burnett, and passed the state bar in june 1894 and federal bar on july 17, 1901. on january 29, 1892, he married esther hall. a republican, he also served as president of the county's bar association.