I have found a John McMullan/McMullen on the 1861; 1871; and 1881 Canada census who may be my John. Still hard to know for sure, though?
This John was a shoemaker. In 1861 in Loughborough, Frontenac, Ontario. In 1871 and 1881 in Brampton, Peel, Ontario where he died in 1882 and have found is buried in the Brampton Cemetery in an unmarked grave.
On those records he shows as married to an Elizabeth/Eliza on the 1861 & 1871 census and a Martha on 1881 census (perhaps the same person as their info seems similar?).
If this is him it would mean he and his first wife, Margaret, went their separate ways. I believe they had a Son, David, in Kingston about 1853.
On the 1861 census John and Elizabeth had a Daughter Mary apparently born in Portland, Ontario about 1858, but she's not on 1871 or 1881 census so may have died early.
Loughborough makes sense to me since it's close to Kingston where David was likely born. Also, Brampton makes sense to me since it's close to where his 2nd Son, William John was from 1867 to the late 1870's in Fergus, Wellington, Ontario.
Would love to know more about this John McMullan. I'd like to know of his marriage record to Elizabeth and/or Martha? Also, birth info on the Daughter Mary could be very helpful.