Although some of the data is incomplete or inaccurate, the Internet Movie Database (IMDb) has information on Karl Hackett: https://www.imdb.com/name/nm0352475/

The Internet Movie Database has a half dozen or so films listed from 1918 - 1925 for William Hackett / William A. Hackett. This appears to be our guy who became Karl Hackett: https://www.imdb.com/name/nm0352509/

There's some 1903 - 1906 and 1924 - 1925 stage credits for "William A. Hackett" at the Internet Broadway Database. Those mid 1920s plays may be for our guy: https://www.ibdb.com/broadway-cast-staff/william-a-hackett-67696

Social Security applications:

Tom McGeeney ordered copies of the Social Security applications for Karl Hackett and his wife Ruby:

The application for Karl lists his name as Wm Karl Hackett and was dated January 30, 1937. Birth date and location were September 5, 1893 in Carthage, Missouri; parents were Wm Germain Hackett and Anna Lee Scott; home address was 5607 Virginia Ave., Hollywood, California. His employer was "Free Lance Picture Actor". (Listing his father as Wm Germain Hackett is some "fudging" by Karl - info indicates his father's name was William G. Germain.)

The Social Security application for Ruby Moore Germain was dated April 1, 1938. She was born 1/1/1898 in Dallas, Texas; parents were Charles Alexander Moore and Katharine Brennan (Moore); home address was 871 N. Oxford Ave, Los Angeles, California. Ruby listed her employer as Alfred C. Watts, 1128 N. Flores, and she began work on January 9, 1938.

More on Karl Hackett can be found in Veterans Administation records, census, draft registrations, and his death certificate:

• World War I draft registration - 23 year old Carl Ellsworth Germain was born September 5, 1893 in Carthage, Missouri, and was living in Carthage. He was married and occupation was "Actor" and working for "Ed Williams": https://www.familysearch.org/ark:/61903/1:1:K3R6-V78
• Veterans Administration Master Index Record for Carl Ellsworth Germain / William Karl Hackett. He was born September 5, 1893; was a Corporal in Company E, 313th Engineers; and served from May 29, 1918 to June 18, 1919: https://www.familysearch.org/ark:/61903/1:1:QRQH-PXZM
• 1940 census and census takers worksheet - 47 year old Carl E. Germain (born Missouri) and 42 year old wife Ruby (born Texas) were renting at 1423 Martel Avenue, Los Angeles, California. His occupation was "actor motion pictures", and in 1939, he worked 12 weeks and earned $1400.00. Wife Ruby was "manager apartment house": https://familysearch.org/pal:/MM9.1.1/K9C6-5N8
• 1942 World War II draft registration - 48 year old Carl Ellsworth Germain was born September 5, 1893 in Carthage, Missouri. Contact was wife Ruby Germain, and they resided at 1423 N. Martel Ave., Hollywood, Los Angeles County, California. No occupation or employer was listed: https://familysearch.org/pal:/MM9.1.1/V4DH-V3G
• Death certificate - Carl Ellsworth Germain, AKA William Karl Hackett, passed away on October 24, 1948 at the National Military Home and Hospital, Veterans Administration Center, Los Angeles. He had been a patient for nearly five months and cause of death was terminal acute bronchopneumonia and cirrhosis of the liver. He was born September 5, 1893 in Carthage, Missouri; parents were William G. Germain and Anna Scott; home address was 127 North Manhattan Place, Los Angeles; his wife was Ruby Germain, and they were divorced; he was a World War I veteran; death certificate informant was "Records of VA Center". Funeral director was the Veterans Administration Center and burial at the VA Cemetery, Los Angeles.
• November 3, 1948 issue of Variety carried a death announcement: "Karl Hackett, 55, film actor, died Oct. 24 (1948) in the Soldiers' Hospital, Sawtelle, Cal., where he had been a patient for nine months. For years he played heavy roles in western films."
• California Death Index mirrors the death certificate and there are dual records for Carl Ellsworth Germain and William Karl Hackett. He was born September 5, 1893 in Missouri, mother's maiden name of Scott, and he passed away October 24, 1948 in the Los Angeles area:
  https://www.familysearch.org/ark:/61903/1:1:VPFZ-5B8
  https://www.familysearch.org/ark:/61903/1:1:VPQY-7Q1
• The Veterans Administration gravesite locator is at: https://gravelocator.cem.va.gov/. In the search box, enter "Germain" in the last name field and 1948 in the death year. Run the search and you should find a record for:
  GERMAIN, CARL E
  CPL US ARMY, WORLD WAR I (rank is Corporal)
  DATE OF BIRTH: 9/5/1893
  DATE OF DEATH: 10/24/1948
  BURIED AT: SECTION 118 ROW B SITE 19
  LOS ANGELES NATIONAL CEMETERY, 950 SOUTH SEPULVEDA BOULEVARD LOS ANGELES, CA 90049
• California Death Index for his ex-wife, Ruby B. Germain - she was born January 1, 1896 in Texas and passed away June 27, 1974 in the Los Angeles area: https://www.familysearch.org/ark:/61903/1:1:VPH2-BDZ

Find A Grave website has a photo of his grave marker. There you will find two separate records, one for Hackett and one for Germain. Both records show interment at the Los Angeles National Cemetery (military cemetery), Los Angeles, California:
https://www.findagrave.com/memorial/10023715/karl-hackett
https://www.findagrave.com/memorial/80965313/germain

Find A Grave indicates that his ex-wife Ruby Burnette Germain (1896 - 1974) is interred at Forest Lawn Memorial Park, Glendale, California: https://www.findagrave.com/memorial/85407245/ruby-burnette-germain

Based on his military records and grave marker info, Hackett/Germain was in the 313th Engineers, 88th Division, US Army in World War I. That unit was formed in August-September, 1917, and the draftees came from Minnesota, Iowa, Dakotas, Illinois, Missouri and Nebraska. That unit did serve in France during the last few months of World War I. One of the websites below mentions that the Division began moving overseas from Camp Dodge, Iowa on July 25, 1918, and the Armistice ending the war was November 11, 1918. There's more about the 88th Division at:
http://historian.itgo.com/Index.htm
http://iagenweb.org/greatwar/units/88Div.htm

With info that Hackett came from Carthage, Missouri, I sent several e-mails to the Powers Museum in Carthage, Missouri asking for info on him. Never received a response. When I checked in 2021, the Powers Museum website was no longer working and newspaper articles reported that the museum closed in 2021 due to funding issues and Covid: https://carthagenewsonline.com/news/powers-museum-closing/

• February 12, 1918 - Warren Evening Mirror (Warren, Pennsylvania) newspaper article:
  "... Mary's Ankle, the brilliant New York success by May Tully, which Mr. Woods will present at the Library theater on Thursday evening with the original New York cast and production."
  "The play is effectively mounted and acted by a notable company, including ... Karl Hackett ..."
• January 7, 1928 - lengthy article in the Zanesville (Ohio) Signal newspaper, including a mention of Ruby Moore who may have been Hackett's wife. Also mentioned is Katherine Moore (spelled KathErine, not KathArine), probably Ruby Moore's mother:
  "Life has been unusually sweet for Karl Hackett since he left Zanesville and he has enjoyed unlimited success. After leaving this city he went to California where he appeared as leading man in motion pictures for four years, playing a number of times opposite Anna Q. Nilsson, who has been so popular with local theater patrons. He was presented in the old Pan American movies and later joined the 101 Bison, Goldwyn and Shamrock companies, most of the latter organizations films being released as Arrow productions. He left the movies to replace Bert Lytell in Mary's Ankle at the Bijou theater on Broadway, where he was acclaimed a tremendous success. After these lengthy engagements he organized his own stock company with which he has been touring the East for several seasons and is now bringing his newest offering back to Zanesville where he has so many admirers. Hackett plays leading roles and comedy parts in his own company and also directs all the productions."
  "There are 25 people with the Hackett Players ..."
  "Numbered among his 15 principals are ... Ruby Moore ... Katherine Moore ..."
• January 21, 1928 ad in the Zanesville (Ohio) Signal newspaper: theater ad for the "Karl Hackett Players" opening January 21, 1928 at the Weller Theater in Zanesville, Ohio in the play It Pays To Advertise. The ad also mentions that the next plays by the "Hackett Players" will be Baby Mine followed by Little Johnny Jones.
• February 3, 1928 article in the Times Recorder newspaper (Zanesville Ohio): article headline reads "Hackett Company Pleases At Weller".
  Other comments in that article include: "Karl Hackett and his company know what the people want and are giving it to them at the Weller theater in Little Johnny Jones this week.", and, "In the role formerly played by George M. Cohan himself at the Weller theater, Karl Hackett is scoring a personal hit with the patrons."
• February 8, 1928 article in the Newark (Ohio) Advocate and American Tribune newspaper: article headline reads "Karl Hackett and Company To Play Here". The "Karl Hackett Players" open in Newark, Ohio (February 8 to February 20, 1928) in same plays as mentioned in the Zanesville (Ohio) Signal newspaper above. Article includes a large photo of Hackett.
• February 20, 1928 article in the Newark (Ohio) Advocate newspaper: article headline reads "Karl Hackett relinquishes management of vaudeville players" and "Following the cancellation of several engagements Karl Hackett of the Hackett Players called his company together Saturday night and relinquished the management."
• September 3, 1929 ad in the Southtown Economist (Chicago) newspaper: typical theater ad announcing that the Elliott Players open Saturday, September 7, 1929 at the National Theater on South Halsted Street, Chicago, Illinois in Grant Mitchell's Hilarious Comedy The Whole Town's Talking. "Mr. Karl Hackett" is listed in the ad as a member of the Elliott Players, "the Foremost Dramatic Stock Company in Chicago".
  
  1922 in Evansville, Indiana.
  
  1923 in Fort Wayne, Indiana.
  
  1928 in Zanesville, Ohio.
  
  1931 in Sioux Falls, South Dakota.

  January 30, 1928 Zanesville, Ohio Times Recorder has a large photo of Hackett who was starring in the play "Little Johnny Jones" at the Weller Theater: https://www.newspapers.com/article/the-times-recorder-karl-hackett-portrayi/171725536/