Greene's law practice was so successful that he was appointed to be a Justice of the Iowa Supreme Court in 1847, to fill the vacancy left by Justice Thomas Stokeley Wilson. He served from November 1, 1847 to January 9, 1855. He was also the reporter of the Supreme Court, so he compiled the decisions of the court in the four-volume G. Greene Reports. In 1849, he became one of the founders of Cedar Rapids, Iowa by surveying and laying out the site of the town. He began living in Cedar Rapids in 1851. He also contributed to the financial development of the city. During the Panic of 1857 he was connected with the management of nine banks in the city.