In the spring of 1838, Greene moved to the Territory of Iowa. In Davenport, Iowa he was employed by David J. Owen, who was making a geological survey of the territory. He spent six months surveying, which helped him gain a better knowledge of the lands. After this, he moved to Ivanhoe, Iowa and taught school while continuing his law studies. In 1840, he was admitted to the bar in Iowa City, Iowa. He moved to Marion, Iowa, and began practicing law. That year, he was chosen to represent Cedar, Jones, and Linn counties in the Council of the Third Legislative Assembly as a Democrat. He was re-elected to the following year to the Fourth Legislative Assembly. In 1845, he moved to Dubuque, Iowa, and became the editor of the "Miners' Express". In Dubuque he began practicing law again, this time in partnership with J. J. Dyer.