"I just don't see Gary Johnson as a serious candidate and I think Americans are realizing that," McMullin said Friday on CNN's "At This Hour."
"He has no knowledge of foreign policy, he spends his time advocating for legalized prostitution and for a drug culture here in America, rather than dealing with problems that are really big like our economy and our national security and government reform," McMullin said.
McMullin, who launched his longshot White House last month, challenged Johnson and Green Party nominee Jill Stein this week to an undercard presidential debate, saying voters "deserve as many options as they can get."
Candidates must reach 15 percent support in national polls to make the stage for the presidential debates, which begin later this month.
Johnson has been the closest third-party candidate to that, polling around 9 percent in national polls. McMullin has been around 1 percent in surveys that have included him.
McMullin's latest comments came after Johnson sought to clean up his gaffe Thursday, when he appeared not to be familiar with Aleppo, the largest city in Syria that has been at the center of the refugee crisis and ongoing conflict.