There are several claims of coinage.
(Oxford English Dictionary)
1947 N.Y. Herald-Tribune 16 Feb. 2/7 (heading) 47,000 subway '*litterbugs' pay $107,000 in fines in 1946 drive.
26 February 1979, AP--Alice Rush McKeon, a conservationist and roadside beautification advocate believed to have coined the word "litterbug," is dead. She died Saturday at her home here at age 94. Mrs. McKeon first used the word litterbug to describe random discarding of trash in a nationally circulated booklet on conservation, "The Litterbug Family," in the 1930s.
5 February 1998, CNN Today--The cartoonist who created the subway signs years ago, Amelia Opti Jones, also known as Oppy, started in the 1940s. Her son survives. She coined some words. Interview-Bill Jones, Oppy's son, says "litterbug" has become part of the language.

