Local Authorities took Census returns at different times and for different reasons,
until the 1840's.
These were basically a "Heads of Household" type of return,
but none for Lennox & Addington are known to have survived..

Provincial Authorities took a Census of Inhabitants in 1842 and again in 1848.
These also were a 'Heads of Household' type of return,
and several have been Indexed.

Nominal returns, where information about each person was gathered, were taken
in 1851 and 1861,
and Agricultural Schedules for some parts of the counties survived to be microfilmed in 1955.

None of these records are available for online Index searching.

Following Confederation in 1867, the first complete national Census was taken in 1871.
This enumeration gathered information about each person, including
name, age, occupation, religious preferences, etc.

The Ontario Genealogical Society has Indexed the Ontario portion of the 1871 Census,
and the names and some particulars about the various
Heads of Households and other 'unconnected' or Stray people that were
enumerated in the Province in the spring of 1871 are
searchable through National Archives files.

Click here to access the entire 1871 Census (Ontario) Index database
(but remember: the Index only lists 'Heads of Households and Strays'.....).
An introduction to the database, with links to the search engine is located here.

The pages referenced contain all of the information neccessary to locate individual 'Heads of Families'
on specific reels of National Archive microfilm.
These films are available through Inter-Library Loan.

Census Enumeration Districts of Ontario, 1871: An absolutely stunning scanned image of an original map.
Lennox & Addington County was contained in two different Enumeration Districts - 63 (Lennox) and 64 (Addington).

The National Archives (of Canada - in Ottawa, Ontario) holds microfilm copies of
those Census returns taken by Provincial and National Authorities.

In addition to the National Archives holdings,
The Government Documents Unit at Queen's University in Kingston
(located a little to the east, in Frontenac County
holds a complete set of all available Canadian Census returns.
The microfilm reels are in open cabinets (but are not available through ILL)