Since pre-1970 Directors Guild Award finalists often consisted of more than five directors, it was impossible to get an exact match for the DGA's and the Academy's lists of nominees. In the list below, the years before 1970 include DGA finalists (DGA) who didn't receive an Academy Award nod and, if applicable, those Academy Award-nominated directors (AMPAS) not found in the — usually much lengthier — DGA list. The label "DGA/AMPAS" means the directors in question received nominations for both the DGA Award and the Academy Award.
The DGA Awards vs. Academy Awards list below goes from 1948 (the DGA Awards' first year) to 1952. Follow-up posts will cover the ensuing decades. The number in parentheses next to "DGA" indicates that year's number of DGA finalists if other than five.
It should be noted that for a number of years, the DGA/AMPAS eligibility periods didn't exactly match. As a result, movies eligible for the DGA Awards one year would be eligible for the Oscars the next — or vice-versa.
For instance, Joseph L. Mankiewicz became the first DGA Award winner — for the year 1948 — for the drama A Letter to Three Wives, an early 1949 release that would earn him the Best Director Oscar for that year (at the 1950 Academy Awards ceremony). Also, Carol Reed's The Third Man was shortlisted by the DGA in 1949, but its Oscar nomination came out in 1950. Additionally, Jose Ferrer won the 1950 Best Actor Oscar for Cyrano de Bergerac, which would earn director Michael Gordon a DGA Award nomination the following year.
I should also note that Directors Guild members could vote for the Best Director Academy Award nominations until 1956. Discrepancies in the DGA/AMPAS nominations during that period, such as Oscar but not DGA nominations for John Huston (The African Queen and Moulin Rouge) and David Lean (Summertime), could be related to eligibility rules (all three aforementioned titles, for instance, were either British or Anglo-American productions — though The Third Man was shortlisted by the DGA), screening availability (both Huston films were last-minute releases), or to different methods of tabulating votes. The Academy uses the notorious preferential voting system.
Source for the DGA nominations: IMDb.
Joseph L. Mankiewicz, A Letter to Three Wives*
Howard Hawks, Red River
Anatole Litvak, The Snake Pit
Fred Zinnemann, The Search
* DGA Award eligibility extended into early 1949
Mark Robson, Champion
Alfred L. Werker, Lost Boundaries
Carol Reed, The Third Man*
Robert Rossen, All the King's Men
* DGA Award eligibility extended into early 1950
Vincente Minnelli, Father's Little Dividend
Carol Reed, The Third Man (see 1949 DGA nominees)
George Cukor, Born Yesterday
John Huston, The Asphalt Jungle
Joseph L. Mankiewicz, All About Eve
Billy Wilder, Sunset Blvd.
Michael Gordon, Cyrano de Bergerac
Henry King, David and Bathsheba
Laszlo Benedek, Death of a Salesman
Anatole Litvak, Decision Before Dawn
Richard Thorpe, The Great Caruso
Mervyn LeRoy, Quo Vadis?
George Sidney, Show Boat
Alfred Hitchcock, Strangers on a Train
John Huston, The African Queen
Vincente Minnelli, The Bad and the Beautiful
Howard Hawks, The Big Sky
Charles Vidor, Hans Christian Andersen
Michael Curtiz, I'll See You in My Dreams
Richard Thorpe, Ivanhoe
Charles Crichton, The Lavender Hill Mob
Hugo Fregonese, My Six Convicts
Albert Lewin, Pandora and the Flying Dutchman
George Cukor, Pat and Mike
Akira Kurosawa, Rashomon
George Sidney, Scaramouche
Stanley Donen and Gene Kelly, Singin' in the Rain
Henry King, The Snows of Kilimanjaro
Elia Kazan, Viva Zapata!
John Huston, Moulin Rouge
[Continues on next page. See link below.]