Johnson, Hideki Matsuyama and Kevin Chappell all carded opening rounds of 66 at East Lake as McIlroy recovered from three-putting the first.
Johnson will secure the $10m (£7.7m) prize money on offer in the FedEx Cup if he wins in Atlanta, with a top-five finish likely to be enough.
McIlroy must win to have any chance.
Johnson - the US Open winner - birdied three of the first six holes and then the last after a 359-yard drive to move into a three-way tie for the lead.
