French ground operations began early yesterday, said Admiral Edouard Guillaud, the French military chief of staff.
France's defence minister said soldiers were heading away from the relative safety of the capital towards rebel strongholds in the north of the West African former French colony.
Five days of airstrikes have done little to erode the Islamist gains, which some in the West fear could turn the region into a launching pad for terrorist attacks.
The ground assault reversed France's earlier insistence it would provide only air and logistical support for an intervention led by African troops.
"Now we're on the ground," Admiral Guillaud said. "We will be in direct combat within hours."
On Tuesday, France announced it was increasing the number of troops from 800 to 2500. The offensive was to have been led by thousands of African troops pledged by Mali's neighbours, but they have yet to arrive, making it increasingly apparent that France will be leading the attack.
A French military spokesman said yesterday that the Islamists had managed to seize more territory despite the air assault because the fighters were embedded with the population, making it difficult to bomb without civilian casualties.
Admiral Guillaud said the militant groups had a history of taking human shields and France would do its utmost to make sure civilians were not targeted.
"When in doubt, we will not fire," he said.
Supplies for the French forces arrived in a steady stream yesterday, part of the enormous logistics operation needed to support thousands of troops in the baking Sahara sun.
Although at least 13 countries have offered support to the mission, only France so far has soldiers there.