It said its forces had "more likely than not" killed 30 other civilians elsewhere in Syria in recent months.
Local monitoring groups, however, said at least 56 civilians had died in the Manbij offensive alone.
The coalition estimated 173 civilians have been killed by its air strikes since the beginning of the operation.
Its attack on Manbij five months ago was designed to aid advancing Kurdish military forces, with the aim of driving IS militants from the city.
An alliance of Kurdish and Arab fighters called the Syria Democratic Forces (SDF) began an operation to capture Manbij at the end of May, and cut critical supply routes to the IS-held city of Raqqa.
The town was eventually captured in August, after a 73-day battle.
In its December casualty report, the US-led coalition contradicted humanitarian groups when it said only 24 civilians had lost their lives in the Manbij attack, along with nearly 100 IS militants.
The report said all seven strikes for which the coalition admitted fault "complied with the law of armed conflict" and it was "unfortunate" that civilian casualties had happened. Those attacks happened between March and October 2016 in both Syria and Iraq.
It rejected 12 other reports of fatal strikes on civilians, and three incidents in October are still being investigated.
In November, the United States also admitted to carrying out an attack in Deir al-Zour, in eastern Syria, which killed dozens of soldiers fighting against IS, which it said was a "mistake".
The attack stopped when Russia informed the US it was hitting Syrian army forces.
Russia said the attack killed at least 62 Syrian troops, and Syrian President Bashar al-Assad insisted that the deadly air strikes were intentional.
The US-led coalition fighting so-called Islamic State (IS) has admitted to causing the deaths of 24 civilians near the Syrian city of Manbij in July.