He also spoke only in Italian - beginning with "buon giorno" (Good day) and ending with "buon pranzo" (Have a good lunch) - instead of greeting the faithful in several languages as his last few predecessors had done.
More than 150,000 people cheered on the new pope in St. Peter's Square.
The pope did tweet in English and other languages, saying: "Dear friends, I thank you from my heart and I ask you to continue to pray for me."
Vatican spokesman the Rev. Federico Lombardi said it was likely Francis would be sticking with Italian, a language he's comfortable with. Lombardi left open the possibility that other languages would be used in the appearances with the public in the future.
In just five days, Francis' straightforward, spontaneous style has become the immediate hallmark of his papacy.
Earlier on Sunday, he made an impromptu appearance before the public from a side gate of the Vatican, startling passers-by and drawing cheers, before delivering a six minute homily - brief by church standards - at the Vatican's tiny parish church.
Before he entered St. Anna's church to celebrate Mass, he shook hands with parishioners and kissed babies.
After Mass, he waded into the street just outside St. Anna's Gate. As the traffic light at the intersection turned green, Francis stepped up to the crowd, grasping outstretched hands. The atmosphere was so casual that several people even gripped Francis on the shoulder.
A few minutes later as the traffic light turned red, Francis ducked back inside the Vatican's boundaries to dash upstairs for the window appearance from the papal apartment in the Apostolic Palace.
The studio window was opened for the first time since Francis' predecessor, Benedict XVI, gave his last window blessing on Sunday, Feb. 24. Four days later, Benedict went into retirement, the first pontiff to do so in nearly 600 years.
The Associated Press contributed to this report.