[4] In 2000, Cantwell was the Democratic nominee for the Plymouth and Norfolk State Senate seat, and was narrowly defeated by the Republican incumbent Robert Hedlund.

[8] Cantwell ran for the 4th Plymouth District State Representative seat in 2008 following the retirement of the incumbent, Democrat Frank Hynes.

Cantwell won a 4-way primary race to face Marshfield independent candidate John Valianti in the general election.

Cantwell's work in the Legislature spanned a variety of issues, including cybersecurity[10] and supporting expanded "buffer zones"[11] for veterans' funerals protested by the Westboro Baptist Church.

[26] For his work on coastal issues, Cantwell was recognized as the Massachusetts Marine Trades Association's 2016 Legislator of the Year.

[27] Cantwell is noted for his work to combat the opioid epidemic, pushing for increased access to recovery coaches[28] [29] and co-chairing the Promote Prevent Commission[30] created by his legislation.

[37] When two homes in Scituate burned down after floodwater contacted electrical equipment[38] that was still live due to a delayed response from National Grid,[39] Cantwell was among the first to call for a Department of Public Utilities investigation,[40] urged DPU to impose fines,[41] and fought to ensure those fines would be returned to affected towns.