Humayun's Tomb, completed in 1570 by the Islamic Mughal dynasty that ruled most of northern India for more than three centuries, is a UNESCO World Heritage site and one of the biggest draws for visitors to the Indian capital.
About 1,500 craftsmen have worked on the tomb and its onion-shaped white dome since 2008, restoring it to its 16th-century glory in a project funded largely by the Aga Khan Trust.
Project director Ratish Nanda said the work had relied on India's rich but increasingly neglected craft industry, while occasionally requiring imported skills for tasks such as traditional plaster and tile-making.
"Until the 18th century, Humayun's Tomb was kept in good nick. But with the decline of the Mughal empire, neglect set in," he told AFP.
"It's a huge building and it had been badly mutilated by really inappropriate repairs."
The tomb and its gardens have been partly open to visitors during the refit. It will be inaugurated later Wednesday by Prime Minister Manmohan Singh and the Aga Khan, a Swiss-born philanthropist and Muslim spiritual leader.
"Humayun's Tomb was a stroke of genius. It is the first of the grand dynastic mausoleums that the Mughals built. There is no precedent," Nanda said. "This is the model and the precursor of the Taj Mahal."
Though the Humayun's Tomb restoration is a success story, funded by charitable donations, activists say India's own state-run Archaeological Survey of India lacks the skills and funds to inadequately care for the country's heritage.