In 2008, when Democratic State Representative Tommy Waters retired and left the District 51 seat in the House of Representatives open, Lee won the three-way September 20, 2008, Democratic primary against Ikaika Anderson and Shaun Christensen with 2,292 votes (54.0%),[11] and won the November 4, 2008, general election with 5,885 votes (61.9%) against Republican nominee Quentin Kawananakoa.

In the midst of the campaign leading up to a legislative special session to decide the issue, Lee received a death threat for his role frequently advocating for same-sex marriage in the media.

On November 12, Lee delivered remarks on behalf of the House of Representatives at a ceremony in which Governor Neil Abercrombie signed the bill into law, making Hawaii the 15th state to legalize same-sex marriage.

[19] In 2015, Lee authored Act 226 allowing transgender individuals to change their birth certificates to reflect their gender identities as recommended by the American Medical Association.

[20] In 2016 he authored Act 135 barring health insurance companies from discriminating against transgender individuals by unfairly denying them medical coverage.

[22] In 2013, Lee authored Act 112 which created a new law requiring super PACs to disclose their top donors in all elections advertising.

[32] After months of investigation in which it became clear NextEra Energy Inc. would pass an excessive $30 billion in costs to consumers, Lee became the first Legislator to publicly oppose the acquisition.

[33][34] Lee led the opposition amongst state legislators and county leaders, organizing support from over 40 other elected officials from both parties.

[52] In 2018, Lee authored and passed Act 15, which made Hawaii the first state legally committing to a zero-emissions economy and statewide carbon neutrality by 2045.

[2][53] He also authored Act 17, which ensures all projects prepare for sea level rise by requiring all environmental impact statements to consider its implications.

[54] Lee has also passed legislation making Hawaii the first state to commit to ensuring basic financial security for all families and begin exploring a guaranteed income for all residents.

[55] In November 2017, Lee, along with Rep. Sean Quinlan, became the first elected officials in the United States to raise concerns about predatory online gaming practices involving loot boxes encouraging minors to gamble after the Star Wars Battlefront II monetization controversy.