Rennie was born in Sunderland, Tyne and Wear, England. When he was four years old, his Scottish parents relocated the family from England to Canada. Rennie was raised in Alberta, as the second of three boys. He began work in university radio, and discovered acting while in Edmonton at the age of 25. For roughly a decade through his late teens and twenties, Rennie struggled with alcoholism. However, in the summer of 1993, a dramatic event led him to change. While in a rough Vancouver bar, Rennie and a construction worker got into an altercation. He 'flipped the bird' to the construction worker, who punched his fist through a plate glass window, and drove a shard of glass into Rennie's left eye and retina. Rennie has had roles in a variety of Canadian television series, including Twitch City, Da Vinci's Inquest and Due South. He has also been in a number of movies, including Double Happiness (as the love interest of Sandra Oh), Curtis's Charm, Hard Core Logo, eXistenZ, Memento, The Five People You Meet in Heaven and Flower & Garnet. He has a recurring role in the reimagined Battlestar Galactica as the Cylon known as Leoben Conoy.