Niall mac Cailein (died 1316), also known as Sir Neil Campbell, was a nobleman and warrior who spent his life in the service of King Robert I of Scotland, His Gaelic name means "Niall, Colin's son" since he was the son of Cailean Mór. His services to the King elevated the Campbells into the higher ranks of the Scottish nobility.
By later Campbell tradition, Niall was the elder son of Cailean Mór; however, contemporary evidence seems to suggest that his brother Domhnall enjoyed this distinction. Niall's earliest appearance in the sources occurs in 1282 on a witness list to a royal charter in favour of Cambuskenneth Abbey. Niall disappears for 20 years, unless the "Master Niall" active in the service of the then Earl of Carrick, Robert, in the 1290s can be identified with Niall mac Cailein. This seems likely, because one official source styles him Mestre Neel Cambell. Another of the sources for "Master Niall" tells us that he came from the "county of Ayr"; this would tie in with the known background of the Campbells of the era, and with Niall's later affiliation with King Robert. In 1293, Niall was sent to Norway to deliver personal items to Robert's sister, Isabella Bruce, Queen of Norway. In 1296, this Master Niall swore fealty to King Edward I of England.
The Niall mac Cailein who appears again in the source in 1302 was still in the service of the English crown. Until 1306, he remained on the side of the officially Bruce-backed English regime. Niall served in the warband of Richard Óg de Burgh, 2nd Earl of Ulster and in the "English" army which besieged Stirling Castle in 1305. Niall and his brother Domhnall were rewarded for their services. In 1302, Niall was given lands in Cumberland. In the same year, Niall and his brother Domhnall received the guardianship of the heiresses of Andrew de Crawford, lord of the Baronies of Loudoun, Lochmartnaham and Draffan. However, Niall and Domhnall, like their lord the Earl of Carrick, were drifting towards rebellion. Niall was at Westminster in 1305, because his rights were being challenged by a knight called Robert Keith. In Spring 1305, Edward decided in favour of Keith, judging "to allow [Keith] to have these children and to distrain Sir Dovenald Chambel and Sir Nel Chambel by their lands and bodies". In the same year, Edward granted some Campbell lands to an English knight, Sir John Dovedale. Such judgments were both a cause and effect of deteriorating relations with the English crown.