Orengo was elected as an MP for Ugenya Constituency as a KANU candidate in a by-election in 1980, making him the youngest MP in Kenya at the time, at age 29. He was elected as a member of parliament for Ugenya constituency in 1992 and served up to 1997, and again in 1997; thereafter he served until 2002. In 2002, he vied for the presidency and lost. In 2005, he joined the Orange Democratic Movement party, teaming up with the right honourable Raila Amolo Odinga after a significant stand-off between the two following a fallout from 2002. In 2007, he vied for the position of Ugenya MP, which he won, enabling him to serve in that capacity until 2013. He was appointed as a Minister of Lands from 2008 to 2013. He was elected and has served in the Senate of Kenya since 2013 as senator for Siaya County.