Three judges in the Phnom Penh court convicted Sonando, who has joint Cambodian-French citizenship, and 13 others of inciting villagers in eastern Kratie province to rebel against the government.
The long-time rights campaigner and critic of Prime Minister Hun Sen was accused of aiming to recruit up to a million people, charges his supporters say were trumped up. Sonando, head of Beehive Radio, had pled not guilty.
Catherine Ashton, of the European Union High Representative, said the conviction "raises severe doubts about the impartiality" of the court.
Human rights groups have accused Hun Sen's Government of granting land concessions to firms moving in to exploit natural resources such as rubber and sugar, and then forcibly evicting land-dwellers.
The EU has come under pressure from activists to freeze a trade initiative that allows Cambodia to export goods tariff-free to Europe, which critics say has contributed to land grabs.