Kashkin was a self-taught musician who had started giving piano lessons by the time he was 13 years old. In 1860 he travelled to Moscow for further study in piano with Alexandre Dubuque. There he met Herman Laroche, Nikolai Rubinstein and Pyotr Ilyich Tchaikovsky.
Nikolai Dmitrievich Kashkin contributed music criticism primarily to the Russian Register (Русские ведомости) and the Moscow Register (Московские ведомости), sometimes under the pseudonym "Nikolai Dmitriev" (Николай Дмитриев).
As a critic, Kashkin would do valuable service in the promotion of Tchaikovsky's music. It was he who provided the epithet "Little Russian" for Tchaikovsky's Second Symphony. Tchaikovsky dedicated the song "Not a Word, O My Friend", Op. 6, No. 2 (1869) to him.
Kashkin published his recollections of Tchaikovsky three years after the composer's death.
He translated music textbooks of L. Busslera, G. Riman, I. Lobe, as well as memories of famous musicians, of the Moscow Conservatory, the Moscow department of the Imperial Russian Musical Society. Published more than 1 500 works. Took part in development of Russian musical culture. He was in friendly and creative relations with P.I. Tchaikovsky, N.G. Rubinstein, S.I. Taneyev and others.