When Sarah failed to get pregnant, as God promised her she would, she decided to take things into her own hands, and give her Egyptian handmaid Hagar to Abraham, so that he could get her pregnant.
“And Sarai Abram’s wife took Hagar her maid the Egyptian, after Abram had dwelt ten years in the land of Canaan, and gave her to her husband Abram to be his wife. And he went in unto Hagar, and she conceived: and when she saw that she had conceived, her mistress was despised in her eyes.” – Genesis 16:3-4
It was this union that led to the birth of their half Egyptian son Ishmael, the father of the Ishmaelites. Abraham was 86 years old when Ishmael was born.
“And Hagar bare Abram a son: and Abram called his son’s name, which Hagar bare, Ishmael. And Abram was fourscore and six years old, when Hagar bare Ishmael to Abram.” – Genesis 16:15-16
After the birth of Ishmael, Sarah began to hate her, and in order to deal with the situation, he sent Hagar and Ishmael away.
“And Abraham rose up early in the morning, and took bread, and a bottle of water, and gave it unto Hagar, putting it on her shoulder, and the child, and sent her away: and she departed, and wandered in the wilderness of Beersheba.” – Genesis 21:14
The very last we hear of Hagar in scripture is when God finds her and baby Ishmael ready to die in the wilderness. God promises her that he will make Ishmael a great nation, and she eventually chooses and Egyptian wife for him. After that she is never mentioned again.
“And God heard the voice of the lad; and the angel of God called to Hagar out of heaven, and said unto her, What aileth thee, Hagar? fear not; for God hath heard the voice of the lad where he is. Arise, lift up the lad, and hold him in thine hand; for I will make him a great nation. And God opened her eyes, and she saw a well of water; and she went, and filled the bottle with water, and gave the lad drink. And God was with the lad; and he grew, and dwelt in the wilderness, and became an archer. And he dwelt in the wilderness of Paran: and his mother took him a wife out of the land of Egypt.” – Genesis 21:17-21