McKinney, 53, held off three rivals to win the party's nomination during its convention in Chicago, Illinois. She picked journalist and activist Rosa Clemente as her running mate.
Green Party spokeswoman Scott McLarty acknowledged McKinney was a "long shot" for the White House, but said, "Every vote that she gets helps the Green Party."
"The United States needs an alternative party," McLarty said. "The narrow two-party system we have right now has not served us very well."