Amicus was created by merger of the Manufacturing, Science and Finance (MSF) union and the Amalgamated Engineering and Electrical Union (AEEU) in 2001. The first Joint General Secretaries were Ken Jackson from the AEEU and Roger Lyons from MSF. Ken Jackson was replaced by Derek Simpson in 2003 and Derek Simpson became the General Secretary of Amicus following the departure of Roger Lyons.
In 2004 both the Union for the Finance Industry (UNIFI) and Graphical, Paper and Media Union (GPMU) merged with Amicus.
Amicus and the TGWU merged in 2007 to form UNITE the Union.