The Amalgamated Society of Engineers was formed in 1851 through proposals drawn up by three unions, the Old Mechanics, the Steam Engine Makers' Society and the General Smiths. However, because some branches of the unions involved failed to ratify the amalgamation the union formed with only 5000 members (less than the membership of the Old Mechanics). Over the following year many of the societies gradually decided on formal amalgamation including the New Society of Millwrights; the Old Society of Engineers and Machinists of London; the London Smiths; the Steam Engine Makers' Society; the United Machine Workers' Asssociation; the United Kingdom Society of Amalgamated Smiths and Strikers; the Associated Brassfounders', Turners', Fitters' and Finishers' Society; the North of England Brassfounders' Society; the Amalgamated Instrument Makers' Society and the Amalgamated Society of General Toolmakers, Engineers and Machinists. By the end of 1851 the number of members had increased to 10481 and the birth of one of the most influential unions in the United Kingdom was complete. However, almost immediately the union was nearly bankrupted through the engineering lock-out of 1852 where employers demanded that workers sign a declaration stating they would not join a trade union movement. After three months the union relented and the men returned to work but from this setback the union recovered quickly (so much so that by 1861 it consisted of 236 branches). The union continued to grow in the following years until in 1920 when the Amalgamated Society of Engineers along with seventeen other Unions joined together to form the Amalgamated Engineering Union.

The history of the union can be traced back to the formation of the "Old Mechanics" of 1826, which grew into the Amalgamated Society of Engineers (ASE) in 1851. The ASE was one of the 'New Model Unions' of the 1850s-1870s. These unions, which also included the Ironfounders, Builders, and Carpenters' societies, rejected Chartism and the ideas of Robert Owen in favour of a more moderate policy based on 'prudence', 'respectability' and steady growth. Great importance was attached to the question of finance, as substantial funds would not only provide maintenance for members involved in strike action, but also help to deter the employers from attacking the organisation. Since its members were skilled and relatively highly paid, it was possible for the ASE to charge contributions of one shilling a week and to build up a fund of unprecedented proportions. In 1852 and 1896, the ASE was involved in extended national lockouts which greatly weakened the organisation. Many local and regional unions joined the ASE in subsequent years and in 1920, after the acquisition of nine fresh member unions, the name of the organisation was changed to the Amalgamated Engineering Union. The AEU continued to grow and absorb smaller unions. Its largest membership growth came during the Second World War when its all-male membership voted to admit women for the first time and 100,000 joined almost immediately. However, the AEU also lost its overseas branches in Australia, New Zealand and South Africa, who became independent unions. The AEU merged with the National Union of Foundry Workers (NUFW) in 1967 and the Draughtsmen and Allied Technicians' Association (DATA) in 1971 to form the Amalgamated Union of Engineering Workers.