You can contact the Johnstown Town Clerk, https://cityofjohnstown.ny.gov/city-clerk.html
and ask if they have any death records in the 1840's.
You can contact the Johnstown City Historian, https://cityofjohnstown.ny.gov/our-history.html
and ask about Indexed newspaper death notices and cemetery inventories.
The Mohawk Valley Library Association, www.mvls.info/lhg/
has bibles, biography, surname and obituary files, cemetery records, etc. Successful Searching!