In 1920, Jackson was a founding member of the Communist Party of Great Britain, although he was not present at the foundation congress. In the early 1920s, he paid visits to Dublin, where he met Constance Markievicz, Charlotte Despard and Maud Gonne, and Moscow, where he was introduced to Joseph Stalin, Sen Katayama and Clara Zetkin, although a planned meeting with Vladimir Lenin was cancelled due to the latter's illness. During the 1920s, he was a major figure in the CPGB, being on the Central Committee from 1924 to 1929 and editor of The Communist and The Sunday Worker.
During the 1920s, Jackson played a significant role in the Communist Party of Great Britain, participating in key events such as attending the party's founding conference (although he was not present) and meeting prominent figures like Constance Markievicz and Joseph Stalin.