Feinstein was elected to the South African Parliament as a member of the African National Congress (ANC) in 1994. As a parliamentarian, Feinstein investigated and exposed a massive and corrupt arms deal that implicated top government officials. He resigned from parliament in 2001, to protest the ANC government's refusal to allow an unfettered investigation into the £5 billion arms deal.

Feinstein lives in London, where he works as a campaigner, writer, and advisor to corruption investigations. He is a founder of Corruption Watch, an organization that monitors and exposes corporate bribery and its impacts on governance, democracy, and development. He is currently creating a web-based resource on the global arms trade for use by civil society groups, watchdogs, elected representatives, activists, and journalists.

His political memoir, After the Party: Corruption, the ANC and South Africa's Uncertain Future, was published in 2007, and became a bestseller in South Africa. Feinstein is chairman of the Friends of the Treatment Action Campaign, a United Kingdom based charity fighting for the rights of people with HIV/AIDS.