The two bitter rivals had looked relaxed as they sat in wingback chairs on a stage beneath a white tent on the lawn of the presidential mansion.
But the chilliness of their relations was clear as Mugabe read out the oath of office for Tsvangirai, who held his hand in the air as he recited: "I, Morgan Richard Tsvangirai, do swear that I will be faithful and bear true allegiance to Zimbabwe and observe the laws of Zimbabwe, so help me God."
"I will well and truly serve Zimbabwe in the office of prime minister," he added.
Mugabe said "Congratulations" and shook hands with Tsvangirai after he signed the pledge that made his investiture official, but there were no smiles, much less back-slapping.
The tent shading more than 300 invited officials, diplomats and dignitaries made the event look more like a wedding than the game-changing political event that Tsvangirai hopes it will prove to be.
Doubt over deal
But this ceremony marked an arranged marriage, demanded by regional leaders to end a year of political turmoil and halt an economic collapse that has left this once-prosperous nation dependent on foreign aid and mired in disease and poverty.
South Africa’s former president Thabo Mbeki, who mediated the unity accord, greeted Tsvangirai at the ceremony.
South Africa sees this moment as a vindication of its so-called "quiet diplomacy", but other countries question if the rivals can work together in government.
The only current heads of state who came were Mozambican President Armando Guebuza and Swazi King Mswati III.
Tsvangirai’s two deputy prime ministers were also sworn in.
Thokozani Khupe, his deputy in the Movement for Democratic Change (MDC), looked Mugabe squarely in the eye as she repeated her oath to "freely give my advice and counsel to the president of Zimbabwe".
The leader of an MDC splinter group drew laughter as he enthusiastically took his oath with his lengthy full name Arthur Guseni Oliver Mutambara.
After the ceremony, the new leaders of the government retreated inside the colonial-era State House, still full of the British decor that Mugabe inherited from the white-minority Rhodesian government when he first took power in 1980.
No chance for celebration
Tsvangirai planned later to head to a stadium to give a speech to his supporters – a chance for celebration that was not found here.
Nearly five months ago, when Mugabe and Tsvangirai signed their power-sharing deal, they appeared triumphant, hands raised together in the air, smiling as regional leaders looked on.
Just days later the deal stalled as the two feuded over key cabinet posts.
The months of stop-and-start negotiations and repeated summits aimed at saving the pact may have made each side more cautious – a feeling shared both at home and abroad over whether their unity government will succeed.