Fellner was born in Río Tercero, Córdoba, and was raised in Villa Palpalá, Jujuy Province, where his father had found work in the Zapla steel mill, the country's oldest and largest.

The young Fellner later relocated to Tucumán, where he earned a juris doctor at the Saint Thomas Aquinas University of the North.

[1] The Secretary of Industry, Juan Schiaretti, named Fellner his Minister of Government during President Carlos Menem's 1993 Federal intervention decree over the governor's post in Santiago del Estero Province.

He resigned the same year, however, amid fallout from a row between Kirchnerists (supporters of then President Néstor Kirchner), to whom Fellner was loyal, and provincial party leaders.

Fellner attempted to change the provincial constitution ahead of the 2007 election so that he could stand for re-election for a third full term.