Born in Prescot, Howarth was educated at the local Huyton Secondary School, the Kirkby College of Further Education, and the Liverpool John Moores University. He went on to study at the University of Salford. He served his apprenticeship for four years from 1966 as an engineer, and then worked as an engineer until 1975 when he moved into teaching.
In 1980 he joined Cooperative Development Services, and in 1982 was appointed the Chief Executive at the Wales Cooperative Centre. He became the Chief Executive of the WalesTrades Union Congress sponsored Centre in Cardiff, a position he held before his election to the House of Commons.
Howarth was elected as a councillor to the Huyton District Council in 1971 and served in its successor the Knowsley Borough Council until 1986, becoming its deputy leader from 1982 to 1983. He was the chair of the Knowsley South Constituency Labour Party for four years from 1981.
The sitting Labour MP for Knowsley North, Robert Kilroy-Silk, resigned from Parliament in 1986 mid term to follow a career with the BBC. In the by-election on 13 November 1986, Howarth was elected with a safe majority of 6,724. He subsequently became MP for Knowsley North & Sefton East in 1997 and Knowsley in 2010 as constituency boundaries were redrawn. In the 2017 general election, he received 85% of the vote, one of the greatest majorities for a British MP since the advent of universal suffrage.
Howarth helped to enact the modern postal voting system. By 1999, the system of postal and proxy voting for those unable to vote at polling stations was seen as cumbersome and complex. Howarth, as Minister of State at the Home Office, chaired the Working Party on Electoral Procedures, which recommended that: absent voting should be allowed on demand and that the application and voting procedures for absent voting should be simplified. The Representation of the People Act 2000 implemented the recommendations. The Representation of the People (England & Wales) Regulations 2001 introduced the changes to the absent voting arrangements from 16 February 2001. The main change was to allow postal voting on demand.
Howarth was appointed one of two temporary Deputy Speakers of the House after the 2015 Queens Speech, until the new deputy speakers were elected on 3 June 2015. Following the 2017 Queens Speech, Howarth again served until the new deputy speakers were elected on 28 June 2017 without standing for the position himself.