Artist's Last Name, First Name. "Title if Any." Publication Name, date of publication, page number or URL if online.
Example #1 (political cartoon retrieved in print):
Block, Herbert. "The American Scene." New York Times, 1 Feb. 1942, E3.
Example #2 (political cartoon retrieved from a website):
Block, Herbert. "What's This About Your Letting the Common People Come in Here and Read Books?" Washington Post, 6 June 1954. Library of Congress. www.loc.gov/exhibits/herblock/classic-cartoons-by-a-master.html#obj2. Accessed 6 Aug. 2018.
APA does not list "cartoon" as a specific example in its manual, but here are some ways to approach it:
If found in an online periodical (ex., newspaper) or periodical database:
Cite as you would a periodical article, but include a description [Cartoon] after the title of the cartoon (if there is one). If the cartoon does not have a title then include [Cartoon] after the date.
Block, H. (1942, February 1) The American scene. [Cartoon] New York Times. Retrieved from http://www.nytimes.com
If retrieved through an online archive, such Herblock's History exhibition through the Library of Congress, indicate the name of the archive.
Block, H. (1942, February 1) The American scene. [Cartoon] New York Times. Retrieved from the Library of Congress' Herblock's
History Exhibition: http://www.loc.gov/rr/print/swann/herblock/
Thanks to Harford Community College for this information on citations