There are numerous stories regarding the origin of Mangal Navgraha. One of the famous stories is that Mangal Navgraha is the son of Vishnu and Bhumi Devi.
Legend has it that demon Hiranyaksha in the form of a pig carried Bhumi (Mother Earth) to the netherworld (Patala Loka). Vishnu then took the form of Varaha Avatar and rescued Her.
When Bhumi Devi was saved from the demon, she assumed her real beautiful form. Vishnu fell in love with Her. They were later blessed with a son, Mangal.
As per Skanda Purana Mangal was born out a drop of sweat of Vishnu and was nursed by Bhumi Devi.
It must be noted here that as per Padma Purana Mangal is the son of Shiva.
Vamana Purana states that Mangal was born when Shiva killed demon Andhaka.