In a White House statement, U.S. President Barack Obama welcomed Sheikh Tamim to his new role, saying Qatar was an "important partner" and paying tribute to his father's leadership.
"The United States looks forward to working with Sheikh Tamim to deepen the ties between our two countries, and to continue our close partnership on issues of mutual interest," Obama said.
Reaction on social media
Social media has been abuzz over the news of the power transfer, with people thanking Sheikh Hamad for his efforts. One tweeter said, "U changed a tiny desert hamlet to a powerful economic empire. Deeply indebted."
Another said, "I have watched Qatar become an economic, sports, education and political leader. #ThankyouHamad for everything. You will always be in our hearts."
UK Foreign Secretary William Hague also chimed in, tweeting, "Historic day for Qatar as the new generation takes over."
Sheikh Hamad was probably motivated by the desire for an orderly transition, having seen close-up what can happen if an older generation clings to power or an impatient crown prince plots a coup, said Ingram.
However, Sheikh Tamim's youth "doesn't mean that he will be more open to democratic reforms," as some might hope, he cautioned.
"His age is largely irrelevant to a conservative mindset. Nothing is proven yet -- we shall have to wait and see what line he will take."