After serving in the British armed forces during the First World War, he combined his theatrical career with work on films, both in Hollywood and the UK.

Knoblock was born in New York City, the second of the seven children of Carl (Charles) Eduard Knoblauch and his wife, Gertrud, née Wiebe.

Knoblock's American-born stepmother, who had attended music conservatory in Leipzig, took the children to Germany, where his older brother was already in school and where the cost of living was lower.

[1] Determined to pursue a theatrical career, Knoblock, in the words of The Times, "settled down to 14 years of hard and unremunerative work, gaining experience of the theatre by acting as well as by writing, adapting and translating plays".

[2] His first dramatic work to be staged was a collaboration with Lawrence Sterner, a revised version of the latter's 1895 play The Club Baby, produced at the Avenue in May 1898,[4] running for 39 performances.

[2] During part of this period he held the post of reader of plays at the Kingsway Theatre, London, where Lena Ashwell and Norman McKinnel were in management together.

[1] In 1909 he returned to Paris, from where he made long visits to Tunis and Kairouan, absorbing the local colour and atmosphere that inspired him to write the play Kismet.

[8] Between the premiere of Milestones and the First World War Knoblock had three more plays presented in London: Discovering America, (1912); The Headmaster (with Wilfred Coleby, 1913); and My Lady's Dress (1914).

After the war Knoblock divided his time between London and Hollywood, where he wrote for the film company of Douglas Fairbanks and Mary Pickford.

[8] In the 1930s Knoblock collaborated on adaptations of novels with Vicki Baum (Grand Hotel, 1931), Priestley (The Good Companions, 1931), Beverley Nichols (Evensong, 1932) and Vita Sackville-West (The Edwardians, 1934).