Onslaught were formed in 1982 in Bristol, England, by guitarist Nige Rockett and drummer Steve Grice.

Vocalist Jase Pope and bassist Paul Hill were then asked to join as Rockett was previously in another band with them called Deranged prior to knowing Grice.

A demo was recorded. A track from the demo was released as the band's first song available to the public; the 7" EP cassette was called "Wessex '83" and was a product of Face of the Subhumans Jase Pope and Paul Hill were soon to be replaced by Roge Davies and Paul Davis, respectively.

The subsequent line-up recorded a demo, "What Lies Ahead", in 1983, which was produced by John Perry of The Only Ones

In late 1985, Sy Keeler joined the band, who assumed the duty of Paul Mahoney.

On 27 March 2018, guitarist Iain GT Davies and drummer Mic Hourihan parted ways with Onslaught to focus on other bands and projects. Nige Rockett said that their replacements had already been chosen, but had not yet been revealed.

He then said that the band would perform a new song from their upcoming album at Bloodstock O.A.. On 16 July 2018, it was announced that guitarist Wayne Dorman and Drummer James Perry had joined the band.

On 18 November 2018, Onslaught posted a teaser video on Facebook, confirming that they are recording their seventh studio album at Grindstone Studios.

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On 8 March 2019, the band released the video for their first song in six years "A Perfect Day to Die", which is "a nod to [their] sadly departed friends from Motorhead who were a major influence for Onslaught over the years.

This song is expected to appear on the band's seventh studio album, due for release later in August 2020 throught AFM records.