The UN and the Syrian Arab Red Crescent sent 37 trucks loaded with "862 metric tons of food, non-food items, education materials, nutrition and medical supplies", it said.
"This is the first time we manage to deliver assistance to Manbij through Aleppo," a government-held city in northern Syria, Fran Equiza, the UN Resident and Humanitarian Coordinator in Syria, was quoted as saying.
Manbij is a former Islamic State (IS) group stronghold now held by a military council affiliated to the Syrian Democratic Forces (SDF), a Kurdish-Arab force supported by a US-led coalition.
One in five people living in Manbij have been internally displaced from other parts of Syria, Equiza said on Twitter. The UN said the delivery would "cover the (food) needs of 50,000 people for approximately 30 days", and provide enough medicine and medical supplies to treat 81,000 people.
According to the World Food Programme (WFP), Syria is experiencing "widespread food insecurity" and 6.5 million people are unable to meet their food needs. Manbij constitutes a major point of contention between Syria's Kurdish minority, which maintains de facto autonomy in parts of northern and northeastern Syria, and neighbouring Turkey.
In December, Ankara threatened to launch a new offensive to dislodge the People's Protection Units (YPG) -- a Kurdish militia that forms the backbone of the SDF, but is considered a terrorist group by Turkey -- from its borders.
Syria's multi-fronted war has killed more than 360,000 people since it began in 2011 with President Bashar al-Assad's regime bloodily suppressing protests.