Jaeckel was born in Long Beach, New York. A short, but tough guy, he played a variety of characters in his 50 years in movies and television and became one of Hollywood's best known character actors. Jaeckel got his start in the business at the age of 17 while working as a mailboy at 20th Century Fox studios in Hollywood. A casting director audtioned him for a key role in the 1943 film Guadalcanal Diary, Jaeckel won the role and settled into a lengthy career in supporting parts. He served in the US Navy from 1944 to 1949, then starred in two of the most remembered war films of 1949, Battleground and Sands of Iwo Jima with John Wayne. Jaeckel's other films include The Gunfighter, Come Back, Little Sheba, 3:10 to Yuma, Town Without Pity, The Dirty Dozen, The Green Slime, The Devil's Brigade, Chisum, Pat Garrett and Billy the Kid, Grizzly, Mako: Jaws of Death, Twilight's Last Gleaming, The Dark Cold River, Starman, Black Moon Rising and The Delta Force 2.

The highlight of Jaeckel's career was in 1971, when he received an Academy Award nomination for Best Supporting Actor for his role in Sometimes a Great Notion. In his later years, Jaeckel was known to TV audiences as Lt. Ben Edwards on the series Baywatch. He also appeared on TV Series Spencer For Hire. Jaeckel died in 1997 after a three year battle with melanoma cancer at the Motion Picture and Television Hospital in Woodland Hills, California.