Both center backs were called up by head coach Gregg Berhalter as replacements for the injured Cameron Carter-Vickers (Celtic) and Chris Richards (Crystal Palace).
McKenzie, 23, has eight caps with the USMNT, while making six appearances for Genk in the Belgian top flight this season. The 25-year-old Palmer-Brown has only featured twice for Team USA, but has played consistently during the current Ligue 1 campaign, logging in just under 700 minutes in eight games with Troyes.
The US will take on Japan (Sept. 23) and Saudi Arabia (Sept. 27) - two World Cup-bound opponents - in their final preparation matches ahead of their Group B debut against Wales on November 21. They'll close out the group stage against England (Nov. 25) and Iran (Nov. 29).
Despite being less than two months away from the tournament, the 26 players called up for the September window aren't locks to make the trip to Qatar, according to Berhalter.
"We have a deeper player pool than we have in the past," he said in an interview with Taylor Twellman during the ESPN broadcast of Sunday's Columbus Crew-Portland Timbers match.
"We're going to have to leave some really talented players off the roster, and those are going to be difficult decisions.
"The message is, this isn't the final roster. There still could be movement. Just do your thing. Keep focusing on your club performance."