Robin Roberts termed it "the dawn of a new day" as she welcomed Stephanopoulos beside her at the "GMA" anchor desk.
He began the program in strong form with a lengthy segment on President Barack Obama's tough calls to bankers to lend more money to homeowners and small businesses to help fix the economy.
In his interview with senior presidential adviser David Axelrod at the White House he also pressed for the current status of health-care legislation.
Then, bringing the exchange to a smooth conclusion, he was interrupted by Axelrod, who jokingly produced a surprise and announced that "your friends in the White House have chipped in to get you this gift: It's an alarm clock permanently set for 3:30 in the morning. So Robin, there's no excuse for him to be late for work."
Stephanopoulos, 48, is making the jump to weekday mornings in New York after seven years based in Washington hosting the Sunday "This Week" political interview show.
At "Good Morning America" he's replacing Diane Sawyer, who next week will take over ABC's dinner-hour "World News" from retiring anchor Charles Gibson.
The appointment of Stephanopoulos, the network's chief political correspondent, was announced only Thursday. But his selection coincides with a mission by "GMA" to shift more toward harder news, particularly in the show's first 45 minutes, and add reports on health, consumer and legal issues,
Beyond that Stephanopoulos' challenge is even clearer: Bring more viewers to "GMA." NBC's rival morning show, "Today," recently marked 14 years of winning each week in the ratings, and its victory margin of 1.5 million viewers was its largest in more than three years, according to the Nielsen Co.
Chris Cuomo, the former "GMA" newsreader who had been thought to be a candidate for Sawyer's job, has left to become co-host of ABC's prime-time "20/20" newsmagazine with Elizabeth Vargas. Juju Chang replaces him as newsreader.
Stephanopoulos, a former White House aide in the administration of President Bill Clinton, moved to a journalism career at ABC News in 1997 as a news analyst for "This Week," where he became host in 2002. Temporarily doing double duty, he will remain at that show until his successor is named - a crucial personnel decision at a time when "This Week" has been drawing critical praise and threatening NBC's long-dominant "Meet the Press" in the ratings.
In the longer term, Stephanopoulos will continue as the network's chief political correspondent and report on politics for other ABC broadcasts. He will also be Sawyer's chief substitute on "World News," the network says.