Malcolm was born in Ashburton, Canterbury, New Zealand. She attended Ashburton College, and graduated from Toi Whakaari (New Zealand Drama School) in 1987. She won an International Actors Fellowship at the Globe Theatre in London for 2003.

Malcolm's first long-running television role was nurse Ellen Crozier in soap opera Shortland Street. She appeared on the show for five years and was nominated for Best Actress at the 1998 TV Guide Television Awards. She was nominated again for her lead role in television feature, Clare, based on the cervical cancer experiment at Auckland's National Women's Hospital which resulted in the Cartwright Inquiry.

In 1999, Malcolm was one of the founding members of the New Zealand Actors' Company along with Tim Balme, Katie Wolfe and Simon Bennett. The company produced and toured a number of successful stage productions throughout New Zealand.

Malcolm won the Woman's Day Readers' Choice Award for Favourite New Zealand Female Personality in 2005, and New Zealand's sexiest woman at the 2007 TV Guide Best on the Box awards.

Malcolm voiced Green Party of Aotearoa New Zealand advertisements for the New Zealand general election, 2008.

Malcolm co-starred in 2010 feature film The Hopes and Dreams of Gazza Snell, playing mother to a family obsessed with go-karting and motorsports. She has also had small roles in movies Absent Without Leave directed by John Laing, The Last Tattoo directed by John Reid, Gaylene Preston's Perfect Strangers, and Christine Jeffs' Sylvia. She had a minor role as Morwen in the second film of the Lord of the Rings trilogy.