Davies was born in Aldershot, United Kingdom on February 27, 1953 and immigrated to Canada in 1968 with her family. She moved to Vancouver, British Columbia in 1969. Before being elected to Parliament, she participated in many grass-roots political organizations in the Downtown Eastside area of Vancouver. She dropped out of university to help Bruce Eriksen found the Downtown Eastside Residents Association (DERA), an influential low-income housing advocacy group. She was instrumental in a campaign to save the Carnegie library which was later converted into the Carnegie Community Centre serving low-income adults. From 1994-1997 Davies was employed by the Hospital Employees' Union.
For 24 years Davies lived in a common law relationship with Vancouver city councilor Bruce Eriksen, who died of cancer in 1997. They had a son, Lief. In 2001, Davies became the first female Canadian Member of Parliament to reveal that she was in a same-sex relationship.
Davies was elected to Vancouver City Council as a member of the Coalition of Progressive Electors (COPE) in 1982 and was re-elected in 1984, 1986, 1988, and 1990. She ran for Mayor of Vancouver with the backing of COPE in 1993, losing to Philip Owen.
Davies was first elected to parliament in 1997 and re-elected in 2000, 2004, 2006 and 2008 and 2011. Previously the NDP House Leader and spokesperson for housing, homelessness and multiculturalism, she became Health Critic in the shadow cabinet of Jack Layton upon the NDP's ascent to Official Opposition status. In parliament she has been a strong supporter of drug policy reform, specifically to halt the criminalization of drug users.
In 2005, during the parliamentary debate on same-sex marriage in Canada, Conservative MP Jason Kenney cited Davies' prior relationship with Eriksen as proof that marriage law doesn't discriminate against LGBT individuals, since a gay person can marry a member of the opposite sex. Davies, who was never formally married to Eriksen, joined other commentators in criticizing Kenney for playing politics with other parliamentarians' personal lives.
In December 2007, Davies received the Justice Gerald Le Dain Award for Achievement in the Field of Law. She was recognized for her "outstanding drug policy reform work" at the 2007 International Drug Policy Reform Conference, hosted by the Drug Policy Alliance and the Criminal Justice Policy Foundation.
Davies faced accusations of anti-semitism stemming from a June 5, 2010, interview in which she suggested that Israel has been occupied territory since 1948. She was criticized for her comments the next day in an Ottawa Citizen editorial. She responded to these criticisms in a letter to the Citizen, which was also posted on Davies' constituency website.
In 2011, it was announced that Davies would serve as health critic for the Official Opposition Shadow Cabinet, while continuing to serve in her role as deputy leader of the NDP.
Davies declined to stand as a candidate for the leadership of the New Democratic Party in 2012, citing her inability to speak French as a factor.
On December 12, 2014, Davies announced that she would retire from parliament at the 2015 general election after 18 years as a Member of Parliament.
In December 2016, Davies was named a Member of the Order of Canada.