He faces the daunting task of shoring up the fledgling peace signed by his illustrious predecessor.
The Sudan People's Liberation Movement/Army (SPLM/A) convened on Monday in the southern Sudanese town of New Site to pick a successor for Garang, who was killed on Saturday in a helicopter crash.
The former rebel movement, which signed a peace agreement in January that ended 21 years of a bloody civil war with the government, logically chose its first deputy chairman Salva Kiir Mayardit.
"The SPLM leadership and the SPLA military command affirmed General Salva Kiir as the chairman of the SPLM and commander in chief of the SPLA," the movement said in a statement after their meeting.
Garang, who had spearheaded the southerners' cause for three decades, had just moved to Khartoum and been appointed first vice-president three weeks ago, when the country started a six-year period of interim rule aimed at sealing the peace deal.
The agreement signed in Kenya in January and the recently drafted interim constitution state that any new chairman of the SPLM/A will automatically take the position of first vice-president.
State Foreign Minister Najib al-Khaeir Abdelwahab confirmed on Tuesday that "it will be Salva Kiir succeeding Garang".
"Salva Kiir has a military background and is a poor orator, ill-prepared to take the position"
The charismatic Garang was the living symbol of his people's struggle for autonomy from Khartoum, but he had also carved for himself an image as the key player in a peace deal observers had hoped could stabilise the restive country.
Kiir, his long-time deputy and a member of the same Dinka tribe, is chiefly a military commander with little diplomatic experience whom some observers predict will struggle to fill Garang's shoes.
He speaks fluent English and Arabic - the language of the north - and is now the only surviving founding member of the SPLA.
"He has been in the SPLA forever," said one fighter, requesting anonymity.
Sudan expert Marc Lavergne was sceptical about Kiir's appointment and highlighted the destabilising effect of Garang's death on an already fragile peace process.
"Salva Kiir has a military background and is a poor orator, ill-prepared to take the position" of Sudanese first vice-president, he said. Other observers were more optimistic.
"It's true he is a military man but he was one of Garang's closest aides and has no alternative to continuing the struggle for a new Sudan," Khartoum-based lawyer and human rights activist Suleiman Ghazi said. The United Nations also heaped praise on Garang's successor.
"He has what it takes to continue on his predecessor's path, until now his declarations are very positive," said Radhia Achouri, spokeswoman for the UN envoy Jan Pronk.
"He is a disciplined man, extremely focused, extremely sharp. A man focused on action, not just words. He listens a lot before making a decision," said a diplomatic source.
Aljazeera's correspondent in Sudan said the reaction in southern Sudan might complicate the scene.
Garang (L), was killed last
Saturday in a helicopter crash
"The political scene has not yet been clear. But Kiir has returned to assume his previous position as SPLM chief of staff, then to succeed Garang. The situation will not be easy to handle as there are armed groups in southern Sudan that might have shown a different response to Kiir's appointment," he said.
Variety of challenges
The new SPLM/A chairman now faces a variety of challenges. Kiir and Garang had their differences, and the new chief will have to assert his authority in southern Sudanese ranks, where some observers charge that Khartoum continues to seek to sow discord.
Three weeks after Sudan has begun a six-year interim period, due to culminate in a referendum on the south's independence, Kiir will also have to rebuild north-south trust, damaged by Garang's death.
"The principal strategy of Garang was to bring people (from north and south) together. I am not sure his successor will manage to do that. Will he manage to have the support of the northern elite?" said a high-ranking SPLM official.
"It doesn't matter whether or not Garang died in a helicopter accident, everybody in the south is convinced that it was an assassination," said Lavergne.
SPLM/A spokesman Pagan Anum said in an interview published on Tuesday by the independent Sudanese newspaper al-Rai al-Am that Kiir could be sworn in as the country's number two in two weeks.