Guterres, a 67-year-old former Portuguese prime minister, takes over from Ban as the ninth United Nations chief on January 1. Ban's second five-year term ends on December 31.
After being sworn in by General Assembly President John William Ashe, Guterres addressed the 193-member nations, saying the world body must work to decentralise and make its bureaucracy flexible.
"It benefits no one if takes nine months to deploy a staff member to the field," he said. "The UN needs to be nimble, efficient and effective. It must focus more on delivery and less on process, more on people and less on bureaucracy."
"UN must be ready to change," he added.
More From This Section
During the special plenary meeting, speakers paid tributes to Ban.
Guterres was unanimously appointed by the General Assembly as the successor to Ban, after the 15-nation Security Council had in October decided by acclamation to send his name to the Assembly for final approval.
The former UN High Commissioner for Refugees had remained the front-runner in the election to choose Ban's successor amid growing calls by civil society and several UN member- states to elect a woman as the Secretary-General.
He had underscored that human dignity, gender equality and fighting the alliance of violent extremists and expressions of xenophobia will be among his priorities as the world's top diplomat.
"I am fully aware of the challenges the UN faces and the limitations surrounding the Secretary-General," Guterres had said in his first address to the General Assembly following his appointment as the 9th Secretary General of the UN.
He had said the dramatic problems of today's complex world can only inspire a "humble approach - one in which the Secretary-General alone neither has all the answers, nor seeks to impose his views; one in which the Secretary-General makes his good offices available, working as a convener, a mediator, a bridge-builder and an honest broker to help find solutions that benefit everyone involved.