By 1911, he had moved to London, where he became a great friend of D. H. Lawrence, and Leonard and Virginia Woolf. He also adored the short-story writer Katherine Mansfield. Although his romantic affection for her was not reciprocated, the two maintained a close relationship in person and in letters until her untimely death in 1923. His friendship with her was also documented in a painting by Beatrice Elvery, Lady Glenavy.

He was business manager of The Adelphi, a prominent literary journal that published works of Lawrence, Mansfield, the young Dylan Thomas, and many other leading lights of early- and mid-twentieth-century English letters after its founding in 1923. But Koteliansky eventually broke with the journal's founder (and Katherine Mansfield's husband) John Middleton Murry. He was an early translator into English (often with the collaboration of Leonard or Virginia Woolf) of works of a number of Russian authors, such as Dostoevsky and Chekhov, and he helped those authors achieve prominence in the English-speaking world.

Koteliansky ('Kot') was a close friend of the artist, Mark Gertler, and they corresponded extensively from 1914 until Gertler's death in 1939.

Koteliansky was a fascinating figure, who made the transition from his origins in a small Jewish shtetl to distinction in the rarefied world of English letters. Although he was not a creative writer himself, he befriended, corresponded with, helped publish, and otherwise served as intermediary between some of the most prominent people in English literary life in the early twentieth century.