She has been a Labor Party member of the Victorian Legislative Council since November 2006, representing Western Victoria Region.
Notably, she was the first woman to become state secretary of the traditionally male-dominated Vehicle Division of the Automotive, Metals and Engineering Union (now part of the Australian Manufacturing Workers Union), having in that role from 1993 to 2006, and was its federal president from 2000 to 2006.
Tierney entered politics at the 2006 state election.
In 2020, Tierney was appointed deputy leader of the government in the Legislative Council.
In June 2022, she was additionally appointed as minister for agriculture.