By 1899, he had become a political writer, creating articles for Pro Coatti, the union magazine L'Edilizia, and the anti-militarist La Pace.
He took up similar activities in Italy, writing for the anarchist newspaper Il Libertario and being heavily involved in the Camera del Lavoro from 1919 to 1922.
Meschi was drawn to Carrara, a hotbed of anarchist and union activity, where his organizing work with Ugo Del Papa led to a two-week strike by quarry workers in the summer of 1911 that resulted in improved conditions.
The pro-war minority were thrown out of or left the USI in September 1914, with the passing of a motion by Meschi which expressed "their trust in the proletariat of all countries to rediscover in themselves the spirit of class solidarity and the revolutionary energy required to take advantage of the inevitable weakening of State forces and of the general crisis caused by the war in order to act to sweep away the bourgeois and monarchist states which have been cynically preparing for this war for fifty years".
In Paris, he helped to found the Antifascist Concentration and the Italian League for Human Rights, also being a member of the Pietro Gori group of anarchists.