After Timmy (Jon Provost) immigrated to Australia, the remarkably long-lived collie spent most of the 1960s (1964-70) working with Forest Ranger Corey Stewart (Robert Bray). The wilderness setting was perfect for new color tv sets, and Lassie got to interact with many different characters, instead of just Timmy and his chums.
After a year by herself (1970-71), Lassie moved back to the boy-rescues, moving onto the ranch run by Keith Holden (Larry Pennell) and his 14-year old son Ron (Skip Burton). She stayed on for 3 years, and finally hung up her collar for good in 1974.
Afterwards Skip (renamed Robert Burton) did not become typecast as a kewpie doll; in fact, immediately after Lassie, he starred in the softcore porn Linda Lovelace for President (1975). Since he was married to 1970s scream queen Karen Black, he also starred with her in Trilogy of Terror (1974).
And several soap operas.
In the 1980s, he went to work on Wall Street.
See also: The Boys of Lassie 1: Jon Provost