With his extremely tall, imposing figure and gray-white hair, character actor John Slattery specialized in utterly convincing portrayals of stoic businessmen, office workers, politicians, and other suits, whenever a film called for one. This typecasting rendered Slattery laudably versatile and prolific; his credits include such multi-genre blockbusters as City Hall (1996), Bad Company (2002), Dirty Dancing: Havana Nights (2004, in the Jerry Orbach role of the disapproving father), and Flags of Our Fathers (2006). On television, Slattery memorably appeared in guest roles on such popular series as Sex and the City (as a politician wooing Carrie), Will & Grace (as Will's older brother), and Law & Order. He found work as a regular on shows like K Street and Jack & Bobby, and appeared in recurring roles on Ed (as a high-school principal) and Desperate Housewives, for which he again played a politician, this time wooing and marrying Gabrielle (Eva Longoria).

In 2007 he was cast as Roger Sterling in the AMC drama Mad Men, a show that would win the Emmy for Best Drama Series multiple times and earned the actor multiple Emmy nominations for Best Supporting Actor in a Drama Series. The show boosted his film career helping to land him the part of Tony Stark's father in Iron Man 2, and playing a mysterious figure in the time-travel thriller The Adjustment Bureau.