When Mon Mothma was born all she heard about in her house was stuff about politics. Her father and mother were deeply involved with the Old Republic. Her father, who was a arbiter-general, and mother, who was a governor, taught her many thing about politics. Then only a few years passed when Mon Mothma joined the Galactic Senate.
Worried about Supreme Chancellor Palpatines policies, Mon Mothma started to employ spies that would follow him around. Despite her forming what would become Rebel Alliance, and what would soon become the New Republic, she was a senator for the Old Republic until the Empire, that was run by Emperor Palpatine, crushed it.
After the collapse of the Old Republic, Mon Mothma became the head of the Rebel Alliance. When the Rebel forces won the battle on Endor, Mon Mothma had to make the stretched out forces of the Rebel Alliance into one fully functional government. After the New Republic was formed Mothma was elected Chief of State for the Republic.
While attending a diplomatic reception Mon Mothma was infected with a nano-engineered virus, which was brought upon her by the Imperial Ambassador Furgan of Carida. No doctor or medic was able to find a way to heal her. The virus slowly wasted her away. While on her deathbed she resigned from Chief of State and passed the position on to Leia Organa Solo. Soon a Jedi healer named Cilghal was able to heal Mon Mothma. When she fully healed she saw the great work Leia was doing and then retired from all political jobs she was involved with. But even after she retired from politics, she always seemed to be around when any big decisions had to be made.
Based upon information from: