The talk show host proposed to McNearney during a trip to South Africa, People reports.
The couple, who were in Kruger National Park when Kimmel popped the question, have been dating for almost three years.
They got together in October 2009 after meeting on Jimmy Kimmel Live, where McNearney was working as co-head writer.
A spokesperson for Kimmel confirmed the engagement but has not given any more details on the proposal or the couple's plans.
Kimmel recently reopened his feud with Jay Leno, admitting that he is "not a fan" of his late night rival.