In 2000, Johnson was an unsuccessful Democratic candidate for the
Vermont House of Representatives. In 2002, she was elected to the House and has been re-elected every two years since then. From 2003 to 2007 she was a member of the House Agriculture Committee. From 2007 to 2017, she was on the Appropriations Committee; from 2015 to 2017 she was the committee's chairwoman. In the November 3, 2020,
general election, unofficial reported totals showed Johnson losing her seat in the two-member Grand Isle-Chittenden district, with the Republican incumbent
Leland Morgan winning 2,768 votes, his nephew and fellow Republican
Michael Morgan winning 2,619 votes, and Johnson trailing with 2,601. Johnson requested a recount, which affirmed Michael Morgan's victory by a 20 vote (2,627 - 2,601) margin. ==Speaker of the House==