French warplanes flew overhead and armed helicopters evacuated the US wounded following the October 4 attack near the Mali border, which is thought to have been carried out by jihadists.
The Pentagon chief has previously praised the less than 30-minute French response time after the attack on the joint US-Niger patrol, and officials say it shows how well the two countries' forces are working together.
But critics have pounced on the fact it fell to France to help American troops as evidence the US military did not have adequate force-protection measures in place, and had failed in its intelligence gathering.
The ambush, which also killed four Nigerien troops, is growing into a possible scandal in Washington as questions swirl about what went wrong, and after it emerged the body of one slain US troop was not recovered for nearly 48 hours -- and only then by a privately contracted helicopter.
"Thank you for your support and for your letter of condolences for our fallen following this attack," Mattis told French Defense Minister Florence Parly.
The French minister, who was on her first visit to Washington, said she would discuss military operations in Syria after the fall of Raqa, the Iran nuclear deal and the anti-jihadist fight in Africa's Sahel region.
"We together won a battle but not completely the war so what comes next is something that I would like very much to discuss with you," Parly said of the anti-Islamic State group fight in Syria.
France has previously called for a diplomatic "contact group" to bring in key players in the Syria drama to oversee a peace process for the war-torn country, although America is opposed to the idea.
Earlier, Parly gave a speech at the Center for Strategic International Studies in which she lamented President Donald Trump's hard line on the Iran nuclear accord and his refusal to certify the deal.
She said letting the deal collapse would sow the seeds of future conflict and be a gift to hard liners.
The United States, which has a growing military footprint in Africa, frequently supports French operations in the Sahel -- notably with aerial refueling to French planes and exchanging intelligence with the old ally.