After the United Kingdom left the European Union in 2020, Long returned as an MLA and was appointed Minister of Justice in the Northern Ireland Executive.

In 2003 Long was elected to the Northern Ireland Assembly for Belfast East,[9] succeeding her fellow party member John Alderdice.

[10] On 6 May 2010, Long defeated Peter Robinson, First Minister of Northern Ireland and leader of the DUP, to become Member of Parliament (MP) for Belfast East in the House of Commons.

Long also became the first Liberal-affiliated MP elected to Westminster in Northern Ireland since James Brown Dougherty in Londonderry City in 1914.

[14] On 10 December 2012, Long received a number of death threats and a petrol bomb was thrown inside an unmarked police car guarding her constituency office.

The election was widely viewed as a success for Alliance, with the party increasing its vote share by 2 percentage points and retaining all of its seats in a smaller Assembly.

[28] Following the collapse of talks to restore devolution in February 2018, Long reiterated her view that the pay of MLAs should be cut in the absence of a functioning Executive.

[29] In March 2018, Alliance launched its 'Next Steps Forward' paper, outlining a number of proposals aimed at breaking the deadlock and Stormont.

[30] At the 2019 Alliance Party Conference, she accused Secretary of State for Northern Ireland Karen Bradley of an "appalling dereliction of duty" over the ongoing stalemate, saying that she had made "no concerted effort to end this interminable drift despite it allegedly being her top priority".

[36][37] In March 2022, Long told the Alliance Party Conference that "some politicians are addicted to crisis and conflict and simply not up to the job of actually governing".

[40] On 28 January, Long announced that she would progress new domestic abuse legislation through the Assembly which would make coercive control a criminal offence in Northern Ireland.

[42] That same month, she announced her intention to introduce unexplained wealth orders in Northern Ireland to target paramilitary and criminal finances.