Cremer was elected as the Secretary of the International Workingmen's Association in 1865 but resigned two years later in 1867, when the organization decided to make women eligible for membership.

[5] From as early as his first unsuccessful run for Parliament in 1868, Cremer had advocated the expansion of international arbitration as peaceful alternative to war for the resolution of disputes.

[5] He was elected as Liberal Member of Parliament (MP) for Haggerston in the Metropolitan Borough of Shoreditch from 1885 to 1895, and then from 1900 until his death from pneumonia in 1908.

Using his platform as an MP, Cremer cultivated allies on both continental Europe and across the Atlantic, including Frédéric Passy, William Jennings Bryan and Andrew Carnegie.

This work includes co-founding the Inter-Parliamentary Union and the International Arbitration League; gaining acceptance for the 1897 Olney–Pauncefote Treaty between the United States and Britain that would have required arbitration of major disputes as the Essequibo territory (the treaty was rejected by the US Senate and never went into effect); and preparing the ground for the Hague peace conferences of 1899 and 1907.