He received an Academy Award nomination for Best Supporting Actor with his role in the 1971 adaptation of Ken Kesey's Sometimes a Great Notion.

[4] He served in the United States Merchant Marine from 1944 to 1946,[5][6] then starred in two of the most remembered war films of 1949: Battleground and Sands of Iwo Jima with John Wayne.

He played the role of Turk, the roomer's boyfriend, in the Academy Award-winning 1952 film Come Back, Little Sheba, with Shirley Booth, Burt Lancaster, and Terry Moore.

Jaeckel appeared in several other Aldrich films, including Big Leaguer (1953), Attack (1956), Ulzana's Raid (1972), and Twilight's Last Gleaming (1977).

Also in 1954, Jaeckel portrayed Billy the Kid in an episode of the syndicated Western anthology series, Stories of the Century, with Jim Davis as the fictitious Southwest Railroad detective Matt Clark.

In 1957, he appeared as Mort Claffey in two episodes, "Paratroop Padre" and "The Light," of the syndicated religion anthology series, Crossroads.

In 1963, Jaeckel played Willie the murderer in "The Case of the Lover's Leap" on CBS's Perry Mason, starring Raymond Burr.

That same year he was among the guest stars on the short-lived ABC/Warner Brothers Western series, The Dakotas and in "The Predators" episode of Have Gun – Will Travel, Season 6 (1962).

Finally in that year, he guest starred in the TV Western Series Gunsmoke in the S8E27 episode "Two of a Kind", playing Irish immigrant mine owner O’Ryan, who was feuding with his partner.

His film career achieved its greatest success in the period 1967 to 1975, in such features as The Dirty Dozen (1967), The Devil's Brigade (1968), Chisum (1970), Sometimes a Great Notion (1971) (for which he received an Academy Award nomination for Best Supporting Actor), Ulzana's Raid (1972), Pat Garrett and Billy the Kid (1973), The Outfit (1973), The Drowning Pool (1975), and Walking Tall Part 2 (1975).

Chisum was a John Wayne vehicle in which Jaeckel, Christopher George and Andrew Prine all co-starred in prominent supporting roles.

The three would re-team six years later in Grizzly (1976) (an amiable "Jaws" ripoff reset in the forest), and Jaeckel and George would team again in another "nature strikes back" story, Day of the Animals (1977).

In 1977, Jaeckel appeared with Donna Mills, Bill Bixby, and William Shatner in the last episode, entitled "The Scarlet Ribbon", of NBC's Western series The Oregon Trail, starring Rod Taylor and Andrew Stevens.