General Dwight Eisenhower was played by Henry Grace.
The interesting thing is... Henry Grace wasn't an actor at all. He was a set decorator who won an Oscar in 1958 for Best Art Direction for the movie Gigi. During his career in film as a set decorator Grace was nominated in the category of Best Art Direction for twelve other movies. He got the part in The Longest Day" because he strongly resembled Eisenhower. Ironically, it's not Henry Grace's voice we hear in the movie, though. The voice of General Eisenhower in The Lontest Day was dubbed by voice-over artist Allen Swift.