By 1198 Burnell's family had bestowed its name on Acton Burnell in Shropshire, where Burnell was born probably in about 1239, as he was close in age to King Edward. His father was probably Roger Burnell, who died in about 1259. He had three brothers, two of whom died fighting the Welsh at the Battle of Moel-y-don in 1282; the third, Hugh, died in 1286. Hugh's son Philip was Robert's eventual heir. Burnell worked as a clerk in the royal chancery, the office responsible for the writing of documents, before moving to the household of Prince Edward, later King Edward I of England. By 1257 Burnell was spending most of his time with the prince and the prince's household. After Simon de Montfort's victory at the Battle of Lewes in 1264, Burnell continued to serve Edward, and was named the prince's clerk in December 1264. As a reward for his service, Burnell was given the prebend of Holme in the diocese of York some time before 1267, and was named Archdeacon of York in December 1270. He also held the office of chancellor to Edward from the time of the Battle of Evesham in 1265 until 1270, when Edward left on crusade.