Born in Mars, Pennsylvania, a small town located about 25 miles from Pittsburgh, he was interested in acting from a young age.

[1] He managed a dairy store in his home town for a year and began taking night courses at the Art Institute of Pittsburgh.

He formed the Fox Chapel Players in Pittsburgh, a stock company composed mostly of former Carnegie students; it lasted one production of Lilliom.

[2] In June 1941 Eythe joined his first professional stock company, in Cohassett, appearing alongside such names as Ruth Chatterton, Nancy Carroll and George Nagel.

[3] Eythe was given a screen-test, and landed a role in the film The Ox-Bow Incident (1943), which co-starred Henry Fonda and Dana Andrews.

Eythe was promoted to leading roles with The Eve of St. Mark (1944), opposite Anne Baxter, from a play by Maxwell Anderson.

He played the juvenile lead in Wilson (1944), Fox's prestige picture of the year; it was a box office disappointment but Eythe's casting in the movie indicated the regard with which he was held at the studio.

[5] Eythe was one of the three leads in a war film, Wing and a Prayer (1944), directed by Henry Hathaway, alongside Don Ameche and Dana Andrews.

[7] He was reunited with Baxter on A Royal Scandal (1945), directed by Otto Preminger (taking over from Ernst Lubitsch) and starring Tallulah Bankhead and Charles Coburn.

Eythe was then given the lead role in The House on 92nd Street (1945) playing double-agent Bill Dietrich (based on William G. Sebold).

He was billed fourth in Centennial Summer (1946), a musical directed by Preminger featuring Jeanne Crain, Cornel Wilde and Linda Darnell.

In 1946, he was one of eight Hollywood actors to give a performance in front of King George VI of the United Kingdom and his consort, Queen Elizabeth.

During the show run he began appearing in TV in episodes of The Philco-Goodyear Television Playhouse ("Dinner at Antoine's", "This Time, Next Year", an adaptation of "The Little Sister", "The Promise").

[1] Eythe was romantically linked with Anne Baxter, June Haver, Margaret Whiting and a male movie actor named Lon McCallister.