Early Life
An Ann Paterson, born in 1754 in Aberdeen, is listed with father of Walter Paterson (
https://familysearch.org/ark:/61903/1:1:XB43-3PK). However, it's hard to think why someone born so far away would have married Benjamin John Johnson in London.  Unless perhaps Walter was a merchant who did a lot of trade from or with London.  In the marriage record (St George Hanover Square) both Ann and Benjamin are noted "of this parish" which doesn't necessarily mean a lot, but at least hints that she was London-based.
Some clues about Ann's parents might be drawn from the name of her younger child, 
Walter Rankin Johnson.  It could be that he took his first name from Ann's father Walter, and his middle name from her mother's maiden name.  Unfortunately, I've found no trace of a marriage record between a Walter Paterson and anyone with the surname of Rankin.
I've put a broad range her birth but in reality it's unlikely that she was much more than 5 years older than Benjamin, putting her date of birth no earlier than 1741.
Death
I have been unable to find a death record for Ann, the issue being that Ann Johnson is simply too common a name to be able to indentify the right record.