French was convicted along with another Norwegian Tjostolv Moland. They were charged with killing their Congolese driver and spying in 2009.
Initially sentenced to death, the men had their sentences commuted.
The men, both former soldiers, denied the charges. They said their driver was shot and killed by gunmen when their car was attacked.
Tjostol Moland, the other man, was found dead in a military prison in August 2013. A Congolese military court found French guilty of his murder.
A Norwegian forensics team sent to Congo as part of Norway’s consular assistance to French told the court that Moland had hanged himself.
The Norwegian government, which denied the two men had been spying for Norway, had been seeking to have them transferred to Norway to serve out their sentences.