Born in Moscow in 1882, Kosloff began his professional ballet career after training at Moscow's Imperial Theater. After graduating in 1901, he began touring internationally with the Diaghilev Ballet Company which he had joined in 1909. While touring with the company, Kosloff began a romantic relationship with fellow company member, the American future set-designer and Mrs. Rudolph Valentino, Natacha Rambova. The affair however, was brief and allegedly tumultuous.
After arriving in the United States in 1909, Kosloff was introduced to influential film director Cecil B. De Mille by the actress and writer Jeanie MacPherson. DeMille was also encouraged to sign Kosloff due to the persistence of his young niece Agnes de Mille, who was an ardent fan of the ballet dancer. DeMille was immediately impressed by the dark-haired young dancer and quickly put Kosloff to work as an actor. Kosloff's first role was in the 1917 DeMille directed The Woman God Forgot opposite the extremely popular American singer and actress, Geraldine Farrar.
By December 1912, Kosloff was reported to be the choreographic director of La Saison Russe, preparing a short run of American premiers of operas and ballets for Spring 1913 in New York, in coordination with Morris Gest. The pre-season announcement promised Mussorgsky's Khovanshchina and Boris Godunov, Rimsky-Korsakov's Sadko and The Tsar's Bride, Anton Rubinstein's Demon, Alexander Borodin's Prince Igor and Mikhail Glinka's Ruslan and Lyudmila.
Kosloff also worked steadily during his acting career as a choreographer and between 1912 and 1916 choreographed several Broadway musicals: The Passing Show of 1915 (1915–1916), A World of Pleasure (1915–1916) and See America First (1916). From 1918 through 1919 Kosloff also appeared on the stage as an actor in the revival of The Awakening.
In early 1923, the Los Angeles Times reported that Kosloff had been offered the throne of the Tatars. He traveled to New York City in February of that year, where he saw his brother and fellow dancer Alexis Kosloff and met with representatives of the Liberal party of Kazan. Fearing the resistance of the Conservative party, Kosloff turned down the offer, saying: "I could be Khan, but it is doubtful for how long. And I decided I would rather be a live motion-picture actor than a dead king!"
Kosloff's career as a film actor spanned the 1920s and Kosloff often appeared as the leading man opposite such well renowned actresses as Nita Naldi, Gloria Swanson, Bebe Daniels and Anna Q. Nilsson. With his dark hair and complexion, the ballet dancer was often cast in more exotic roles, often as a "Latin lover" type, Eastern European prince or noble, or Arabic sheik. Kosloff's acting career often relied heavily on DeMille procuring roles for him in his films. Indeed, the majority of Kosloff's film roles are in DeMille directed films.
Kosloff's acting career came to an end with the advent of sound film. Studio executives were reluctant to cast him in roles because of his pronounced Russian accent. His last substantial on-screen role was as the human embodiment of Electricity in a dance scene in a Zeppelin in De Mille's musical flop Madam Satan. Kosloff's last film role was an uncredited role as a dance instructor in the 1937 Gregory La Cava directed Stage Door, opposite Ginger Rogers, Katharine Hepburn and Adolphe Menjou.
He was the partner with madame Matilda Kshesinskaya who still lived in Paris, France. Since 1917, he consulted motion picture producers like Cecil B. De Mille and one of the last movies he was involved with was, up until his death The Ten Commandments.
After retiring from acting, Kosloff continued to work as a choreographer and opened a successful ballet school in Los Angeles, where choreographer Flower Hujer was one of his students.
On Thanksgiving morning 1956, Theodore Kosloff felt ill and was taken to Hollywood Hospital where he died at the age of 74 and was interred at Valhalla Memorial Park Cemetery in North Hollywood.
He was survived by his estranged wife Madame Maria Baldwin (also spelled Baldina), and a brother Alexis Kosloff from Woodstock, New York, a sister-in-law Madame Baldina, also a ballerina.
For his contribution to the motion picture industry, Theodore Kosloff was given a star on the Hollywood Walk of Fame located at 1617 Vine Street, in Hollywood, California.