The blaze has destroyed a number of homes, dropping ash on the streets of the city in the province of Alberta.
Fleeing residents have caused gridlock on the main road leading from the city, 380km (235 miles) north of Edmonton.
The evacuation from Fort McMurray - which lies in an oil sands region - is the biggest in Alberta's history.
"If you just walk outside, you feel it (ash) falling on you. You see it floating in the air. I can take a broom and brush it off my deck,"  resident Mark Durocher was quoted as saying by the Globe and Mail.
Homes in at least two neighbourhood have been gutted, and the fire has now spread to Highway 63 - the main road into Fort McMurray from the south.
Firefighters are continuing to tackle the blaze, but the local authorities have called for reinforcements, including a water-dumping helicopter.
So far there have been no reports of any injuries.
A huge wildfire has forced the evacuation of about 60,000 people from Fort McMurray - the entire population of the Canadian city.