Lee was the Workers' Party's Deputy Treasurer from 2008 to 2011, the party's Youth Wing President from 2011 to 2012 and the Deputy Webmaster from 2012 to 2014. Since 2014, she has been the Organising Secretary on the party's Executive Council. Prior to her election to parliament, she also served as a legislative assistant to Pritam Singh, who he was now the Secretary-General of the party since 2018, and as an MP of the Aljunied Group Representation Constituency, where she was defeated to the governing People's Action Party candidate, Michael Palmer, with 16,994 to 12,777 votes , received only 1,387 votes ). However, on 12 December 2012, Palmer resigned his Speaker and MP post citing that he had an extra-marital affair against a constituency director from Pasir Ris West division, eventually precipitated a by-election which would be held on January 2013. On the same day, then-Secretary General of WP, Low Thia Khiang announced the party candidacy, and would field Lee to contest the seat again in the forthcoming by-election. At the close of the nomination, Lee faced a rare four-cornered contest between PAP's Koh Poh Koon, the SDA's Lim, and a fourth candidate, the Reform Party's Secretary-General Kenneth Jeyaretnam. On 26 January, Lee won the election with 16,045 votes, with her rivals Koh, Jeyaretnam and Lim, received 12,875, 353 and 168 votes, respectively. This win marked the second time after the 1981 by-election the WP had captured another parliamentary seat after a by-election, and Lee was also the first female candidate to control a SMC. Upon her win, Lee told the media that her immediate priority as Member of Parliament would be to ensure the smooth handover of the Town Council from the ruling PAP. Lee was sworn in as a Member of Parliament on 4 February 2013. In the 2015 General Election, Lee was announced as a candidate who would defend the constituency, and on 27 August, PAP revealed that six-term MP and Deputy Speaker Charles Chong would be the challenger. On 11 September, Lee was defeated in her re-election 48.24% to 51.76%, and thus returning Punggol East SMC to the PAP. Although she was offered a Non-Constituency MP position by-virtue for being the best-scoring losers of the election, Lee declined the offer, the first time someone had done so since 1984. It was later revealed that Daniel Goh, a candidate part of the Workers' Party team for the East Coast GRC, would replace Lee as the last NCMP seat, on 4 February 2016.
Lee is married to Koh Chee Koon, a telecommunications consultant. On 15 January 2014, it was announced that she is pregnant with her first child and on 1 July 2014, she gave birth to a baby girl.
