After leaving the council the following year, she was the Labour Member of Parliament (MP) for West Lancashire from 2005 until her resignation in 2022, when she was named chair of the Mersey Care NHS Foundation Trust.

Next, Cooper contested the 1986 Knowsley North by-election, caused by the resignation of the Labour MP Robert Kilroy-Silk to become a television presenter.

[7] Cooper became the Labour Party's candidate from an all-female short list, in the constituency of West Lancashire at the 2005 general election, following the retirement of the sitting MP Colin Pickthall.

[10] Following her election in 2005, she became a member of the Northern Ireland Affairs Committee, and was part of the successful campaign that stopped the merger of the Southport and Ormskirk hospitals.

[citation needed] On 9 August 2006, The Daily Telegraph wrote that Cooper had written to the Prime Minister's office reporting the viewpoint of some of her constituents expressed to her, that they would be appalled if Baroness Thatcher were to be given a state funeral, as a leader more politically divisive than others of the late twentieth century.

[14] On 12 June 2018, Jack Renshaw, 23, of Skelmersdale, Lancashire, admitted in a guilty plea to buying a 48 cm (19 in) replica Roman Gladius sword (often wrongly referred to in the media as a machete) to kill Rosie Cooper the previous summer.

[19] In June 2021, Cooper introduced a Private members' bill which would give British Sign Language legal recognition and enhance its use in public services.

[21] In September 2022, Cooper announced she accepted a new role as chair of Mersey Care NHS Foundation Trust and would resign as MP for West Lancashire triggering a by-election.

[25] Cooper was appointed Crown Steward and Bailiff of the Chiltern Hundreds on 30 November[26] and formally succeeded Beatrice Fraenkel as chair of Mersey Care NHS Foundation Trust, stating that "representing West Lancashire in Parliament has been the greatest honour of my lifetime".