[1] She is a Member of the Legislative Assembly (MLA) for East Londonderry, having been co-opted to the position after the death of David McClarty in 2014, and won election to the seat in 2016, 2017 and 2022.

In 2008, she began working as a parliamentary assistant for David McClarty, then Ulster Unionist Party MLA for East Londonderry, a role she maintained until her co-option in 2014.

She was nominated to succeed him in his seat in February 2014 after he fell ill.[8] After McClarty's death in April 2014, Sugden was co-opted as the MLA for East Londonderry, becoming one of the youngest MLAs in the Northern Ireland Assembly.

[10] During her first two-year term, she was considered a member of the "naughty corner" in the Assembly for being "rarely quiet and not afraid to stand up to the two major parties".

"[12] Speaking in the Assembly on the November 2015 Paris attacks, she said "it takes a special kind of bastard to inflict that pain on so many," for which she was warned by the Speaker for her use of language.

[19] On 6 January 2017, she reiterated her support for an independent investigation but denied claims that she had unilateral power to call a public inquiry into Foster's actions.

"[20][21] On 9 January, McGuinness resigned as Deputy First Minister, and Sinn Féin refused to nominate a replacement, leading to the collapse of the Executive Office and to the calling of new elections, for March 2017.

[22] After a suggestion that she might quit politics, Sugden announced that she would stand for re-election in January 2017, saying that "I never envisaged that less than a year into my mandate, I would be defending my seat for East Londonderry.

[26] In May 2021 Sugden turned down an invitation from Doug Beattie to join the UUP, preferring to remain as an independent, but did not rule out a move to the party in the longer term.