Martiniuk was born on August 5, 1937 in Toronto. He was educated at Lawrence Park Collegiate Institute, University of Western Ontario, University of Toronto (Political Science and Economics) and Osgoode Hall Law School.
Martiniuk began his career at the municipal level, serving as a school trustee on the Waterloo County Board of Education, an alderman in Preston, and chair of the Waterloo Regional Police Commission.
In the 1995 provincial election, he was elected in the riding of Cambridge over incumbent New Democrat Mike Farnan.
He was easily re-elected in the 1999 election, defeating Liberal Jerry Boyle by over 14,000 votes. In 1998 he was appointed as Parliamentary Assistant to the Attorney General.
The PCs were defeated in the provincial election of 2003, although Martiniuk was again re-elected, this time defeating Boyle by just over 3000 votes. He served as opposition critic for several portfolios including Labour, Consumer and Business Services, Community and Social Services Critic and Seniors.
In 2004, he supported John Tory's successful bid replace Ernie Eves as provincial PC leader. Martiniuk was again elected in the provincial election of 2007 with a plurality of 3243. He served as the critic for Revenue and Seniors. In 2008 he supported Tim Hudak's successful PC leadership bid. In 2011, he introduced the Ukrainian Heritage Bill which sets September 7 as Ukrainian Heritage Day in Ontario to mark the anniversary of the arrival of the first Ukrainian immigrants to Canada in 1891. The bill was passed on March 30, 2011.
On October 14, 2010, Martiniuk announced his decision not to run in 2011 election.
He was a director of the St. John Ambulance, Cambridge United Way, Preston-Hespeler Rotary Club, Trinity Community Table, Preston YMCA, Chair, Conestoga College Advisory Committee on Criminology before entering provincial politics, and served as president of the Cambridge Chamber of Commerce.
He and his wife Christine raised four children. He died in Cambridge, Ontario at the age of 79.