Her dam was the 1934 American Champion Two-Year-Old Filly Nellie Flag, and her sire was the 1930 Epsom Derby winner Blenheim, who had been imported to the United States in 1937 by a syndicate that included Mar-Kell's breeder, Calumet Farm.
Racing Hall of Fame inductee Ben Jones, as a four-year-old, Mar-Kell was named American Champion Older Female Horse of 1943.
[2] At age three her best effort was runner-up to Star Copy in the Lady Baltimore Handicap at Pimlico Race Course[3] but as a four-year-old Mar-Kell came into her own, earning 1943 National Champion honors in her class.
Mar-Kell's most significant wins that year came at Belmont Park in the Top Flight and Beldame Handicaps.
Racing Hall of Fame filly Ruthless who won the 1867 Belmont Stakes in its inaugural running.