Somyuth Phumpakdi was found guilty of arranging the murder of Suwat Wongpiyasathit on June 26, 2001.
Suwat, a local resident, led a protest against the foul smell eminating from the 200-rai (320,000-square-metre) dump in Racha Thewa sub-district of Bang Phli district, Samut Prakan.
Somyuth was an executive member of Racha Thewa administrative organisation. The dump received garbage from Bangkok.
The Appeal Court upheld the death penalty, but the Supreme Court commuted it to life imprisonment because he had confessed when he was arrested and the confession had facilitated the prosecution in the case.
Life imprisonment sentences were upheld for the gunman, his motorcyclist and another accomplice. A former sergeant who had contacted the gunman used by Somyuth died behind bars in 2010.