After retiring, he became the chairman of a financial and strategic advisory firm Grenier joined the CIA in January 1979 and worked field assignments in North Africa, Middle East and Western Europe till 1991.

He served as Special Assistant to the Under Secretary of State for Political Affairs Peter Tarnoff (1993–94) and first Chief of Operations, Counter-Proliferation Division (1994–96).

In early 2006, Grenier was identified in court documents in connection with the CIA leak grand jury investigation and charges against I. Lewis "Scooter" Libby.

[5] In 2009 Grenier was appointed Chairman of ERG Partners, an independent financial and strategic advisory firm solely focused on the security and intelligence sectors.

In the wake of the January 6 United States Capitol attack, Grenier argued in an op-ed for The New York Times and interview with NPR that widespread violence from "the dawn of a sustained wave of violent insurgency within our own country, perpetrated by our own countrymen" may be imminent, describing former President Trump's popularity as "insurgency leadership" in "subversion of the Constitutional order."