McNamara, Robert S.Mc‧Na‧ma‧ra, Robert S. /ˌmæknəˈmɑːrə $ ˈmæknəˌmærə ˈrɒbət $ ˈrɑːbərt/(1916–2009) a US politician who became Secretary of Defense in 1961, and who left his position 1968 because he and President Johnson did not agree about US policy in Vietnam.