He was a Member of the Legislative Assembly (MLA) of British Columbia, representing the riding of Surrey-Whalley from 2005 until 2024.

A member of the New Democratic Party (NDP), he has served in the cabinets of Premiers John Horgan and David Eby.

[7] In July 2017, Ralston was named Minister of Jobs, Trade and Technology in the NDP minority government.

[8] He swapped portfolios with Michelle Mungall in January 2020, becoming Minister of Energy, Mines and Petroleum Resources.

[9] His post was modified to Minister of Energy, Mines and Low Carbon Innovation and Minister Responsible for the Consular Corps of British Columbia in November 2020,[10] and he was appointed Queen's counsel in December of the same year.