Here's a nice one on Young Avenue in Halifax, which is several blocks of large houses (the Oland house is farther down):
Government House is probably the oldest official residence in Canada, cornerstone laid in 1799:
Admiralty House is another interesting one, built around 1812 or so for the admiral of the British fleet when Halifax was one of the main naval bases in the Empire (by ewan.osullivan on Flickr):
I've always liked Keith Hall (in rough shape, sadly) and Wier House:
There are hundreds of large old houses around the city and maybe a few dozen that are true "mansions". Unfortunately, they are kind of hard to photograph systematically since they are very spread out. Many have also been demolished over the years.