Jason Clarke Ted Kennedy Kate Mara Mary Jo Kopechne Ed Helms Joseph Gargan Bruce Dern Joseph Kennedy Jim Gaffigan Paul Markham Olivia Thirlby Rachel Schiff Clancy Brown Robert McNamara Taylor Nichols Ted Sorensen John Fiore Chief Arena Victor Warren Stephen Smith David De Beck Sargent Shriver Matthew Lawler Dun Gifford Thomas Kee Walter Steele Andria Blackman Joan Kennedy Tim Jackson Joseph Kopechne Damien Di Paola Ray LaRosa Gary Tanguay TV Interviewer Tony Viveiros Floor Producer Tamara Hickey Marilyn Richards Nancy O'Brien Neighbor Tom O'Brien Neighbor's Husband Michele Proude Vicki Michael Fennimore Chief of Staff Dustin Tucker Senatorial Aide Paul Pape Dick Drayne (voice) Chris Edgerly Edmund Dinis (voice) Wally Wingert Newscaster (voice) Piotr Michael Newscaster (voice) Bill Mootos Older Reporter Philip Dorn Hebert Jack Crimmins Patrick Sheehan Lawyer Michael Swanson Security Agent Susan Garfield Senate Staffer (uncredited) Paul Bellefeuille Dike House Resident (uncredited) Joe Cali Reporter (uncredited)