Following the 2016 Scottish Parliament election, Callaghan worked as Office Manager to Rona Mackay, the MSP for Strathkelvin and Bearsden.

[13] At the December 2019 general election, she won the seat, only narrowly beating the incumbent MP and Liberal Democrat leader, Jo Swinson.

[22] On her physical return to the House of Commons in February 2022, Callaghan stepped up her calls for proxy voting during a speech in parliament and in a serious of interviews including with the Daily Record.

[23] On 7 June 2022, Callaghan was forced to withdraw her comment in the House of Commons for using the word "lying" in relation to the prime minister, Boris Johnson.

She criticised Johnson for "partying, lying, amending the ministerial code, voter suppression, [and] watering down human rights".

[24] In June 2022, Callaghan's colleague Patrick Grady was found to have committed "unwanted physical touching, with sexual intent" on a junior member of the SNP Westminster Group's staff.

The expert group found that Grady's behaviour was a "significant breach" of the UK Parliament's sexual misconduct policy.

"[26] Her comments had been greeted with applause from colleagues, but Callaghan later issued an apology, describing her remarks as "insensitive, poorly worded and misplaced".

[27] Then First Minister and SNP party leader, Nicola Sturgeon, subsequently told the Scottish Parliament, "I cannot comment on whether [the leaked recording] is an accurate overall reflection of the discussion, but what I have heard suggests that more concern was shown for the perpetrator of the behaviour than for its victim.

[32] During the 2024 UK general election, Callaghan lost in her bid for re-election in the newly formed seat of Mid Dunbartonshire, trailing the Liberal Democrats' Susan Murray by 9,673 votes.