In the June 2016 European Union membership referendum, Johnson voted for Brexit. Later in the year, a by-election was called in the Sleaford and North Hykeham constituency after the resignation of its MP Stephen Phillips following differences between him and the government on Brexit. Johnson was selected by the Conservative Party to contest the seat in the December by-election. In the by-election she was elected as MP with 17,570 (53.5%) votes and a majority of 13,144. Johnson retained her seat in the 2017 general election with 42,245 votes (64.2%) and a majority of 25,237. Following the election, she was chosen to be part of the Health Select Committee and the Environment, Food and Rural Affairs Committee. She served on the Health Select Committee until February 2018. Johnson also chairs the All Party Parliamentary Groups (APPG) for Children who need Palliative Care and Rural Crime.