His parents recognized his artistic skills when he created a portrait of the family dog at the young age of four and he sketched, painted or drawn every day until his death.
He barely escaped death in a railway line reconstruction accident at Hudson, Ontario, in 1936, losing his right arm at the shoulder after falling under a CNR rock car while operating a steam shovel.
The paintings depict many of Canada's principal large animals; grizzlies, black and polar bears, timber wolves, mountain lions, musk-oxen, woodland and barren caribou, moose, pronghorn antelope, dall and bighorn sheep, mule and white-tail deer.
Between 1957 and 1959 Tillenius travelled by pack-horse on a number of trips in the Canadian Rockies and Waterton Lakes with rancher, author and environmentalist friend Andy Russell.
In May 1959 he packed into the Kluane with Andy and Dick Russell to paint and draw grizzly bears, wolves, moose and golden eagles.
Tillenius left for a study trip to Europe in 1962 and was able to view the works of Anders Zorn, Bruno Liljefors of Uppsala, Sweden and the animal painter and illustrator Harald Wiberg.
In 1964 Tillenius joined Ralph Hedlin who was on a writing and photography assignment for Maclean's, and the pair traveled with Inuit by dog team, lived in igloos, and observed firsthand the hunt for polar bears on Southampton Island.
In August of that year, he traveled to Vancouver Island to hunt with Jim Dewar and to choose the environment and paint the background to be depicted in a cougar diorama in Victoria.
Tillenius taught wildlife drawing classes at the Okanagan Summer School of the Arts near Penticton, British Columbia, for ten years until 1978.
In 2005, Tillenius painted two of sixty cement polar bears, each 8 feet (2.4 m) tall and weighing 10,000 pounds (4,500 kg), created as a fundraising project for Cancer Care Manitoba.