Outgoing chief executive officer Sol Trujillo said a replacement to lead the wholesale group "will be announced in the coming weeks".
McKenzie begins her role at the end of the month.
Stewart is the latest high-profile Telstra executive to return to the US.
He joins former corporate affairs boss Phil Burgess, who stepped down last year, chief operating officer Greg Winn who stepped down in late January, and Sol Trujillo, who will end his tenure with the telco at the end of June.
Trujillo credited Stewart's leadership for helping deliver "increases in market share, improved margins, lower churn and high strike rates for sales".
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