More than 1,200 people have been killed in land conflicts in Brazil, mostly in the Amazon region, in the past 20 years, according to the Catholic Land Pastoral, a watchdog organisation monitoring rural violence in the country.
However, only 80 gunmen have been convicted for such crimes and none of those who planned the killings have gone to jail until now.
Two others - Rayfran des Neves Sales and Clodoaldo Batista - have already been sentenced to 28-year jail terms for carrying out Stang's murder, while a third, Amir Feijoli, is in prison for hiding the assailants.
Prosecutors said that Moura ordered the killing because Stang had prevented him and another rancher from taking over land that the government had given to local farmers.
Stang had worked in the region for 30 years defending small farmers who were being forced off their land by large property owners.
"We've waited so long for this verdict," Rebeca Spires, a nun who has worked in Brazil for four decades and knew Stang for 35 years, said.
"This conviction sends a strong message to the other masterminds that the impunity is ending."
Moura was originally convicted in 2007 of the murder, before being acquitted in an automatic retrial. But that decision was overturned last year on a technicality.