The exact date of the birth of Umar is not known. The concensus of opinion, however, is that Umar was born at Mecca around 580 A.D He was younger than the Holy Prophet of Islam by about ten years.
Umar belonged to the Adi clan of the Quraish. It was one of the ten clans of the Quraish who inhabited Mecca.
The pedigree of Umar was: Umar the son of Khattab; the son of Nufail; the son of Abul Uzza; the son of Riza; the son of Ribah; the son of Qurat; the son of Adi; the son of Katb.
The pedigree of the Holy Prophet was: Muhammad (peace be on him) the son of Abdullah; the son of Abdul Muttalib; the son of Hashim; the son of Abd Munaf; the son of Qussay; the son of Kulab; the son of Ka'b.
In the case of Abu Bakr and the Holy Prophet, Murrah in the eighth degree was their common ancestor. In the case of the Holy Prophet and Umar, Ka'b in the ninth degree was their common ancestor.
Among Umar's ancestors, Adi rose to prominence as a diplomat, and the clan came to be known after him. Whenever the Quraish of the day had to negotiate any settlement with any other tribe, Adi represented the interests of the Quraish as an ambassador. Even in the case of disputes among the Quraish themselves, Adi acted as the arbitrator. After the death of Adi the two offices of diplomatic representation and arbitration became hereditary in his descendants.
Umar's grandfather Nufail arbitrated in a dispute between Abdul Muttalib, the grandfather of the Holy Prophet and Harab bin Umayyah over the custodianship of the Ka'bah. Nufail gave his verdict in favour of Abdul Muttalib. Addressing Harab bin Umayyah he said:
"Why do you pick a quarrel with a person who is taller than you in stature; more imposing than you in appearance; more refined than you in intellect; whose progeny outnumbers yours and whose generosity outshines yours in lustre? Do not, however, construe this into any disparagement of your good qualities which I highly appreciate. You are as gentle as a lamb, you are renowned throughout Arabia for the stentorian tones of your voice, and you are an asset to your tribe."
This address is indicative of Nufail's skill in diplomacy and his highly developed sense of judgment.