Harwell ran for office for the first time while she was teaching at Belmont University, running for the 56th District in the Tennessee House of Representatives in 1986.

[10] During the elections, the Republican Party gained control of both the Tennessee Senate and House for the first time since Reconstruction.

[6] Harwell's main focuses have been welfare reform, children's issues, sex abuse laws, victims' rights, and education.

Additionally, she secured the passage of legislation that directed more funds to improve the training of day care workers.

[19] As Speaker, Harwell created a task force in 2017 to hear from experts and those affected by the crisis.

[20] Among the task force's recommendations are adding more TBI agents, starting a pilot program of a recovery center in each grand division of the state, and modernizing pain-management curriculum for healthcare professionals.

During the 2018 session of the Tennessee General Assembly, Speaker Harwell cast the tie-breaking vote in the House Criminal Justice Subcommittee to advance legislation legalizing medical cannabis.

[22] During a GOP candidate debate, Harwell stated that she believed it would be "cruel to withhold that from someone who needs that help".

As speaker, Harwell supported the efforts to end Common Core in Tennessee and to allow for charter schools, and was a leader in the fight for more investment in education.

[19] Harwell has advocated for a renewed focus on literacy in lower grades and more emphasis on how valuable technical school can be compared to college.

Harwell directed the House Government Operations Committee to hold additional hearings on the issue,[27] and asked the State Comptroller of the Treasury to review the testing vendor, Questar.

[19] Second Amendment Harwell has supported and advocated for the rights of gun owners for years as the Speaker of the Tennessee House of Representatives.