Simon Finch was an ancestor of Jem and Scout.  He immigrated from England to the United States due to religious persecution of the Methodists.  He lived in numerous places before settling in Alabama, notably Philadelphia and Jamaica before moving to Mobile and eventurally to Saint Stephens. When he arrived in Alabama he made a lot of money practicing medicine.  However he was unhappy doing this.  Being a strong Methodists, he felt he might be tempted into wrong actions:

"...tempted into doing what he knew was not for the glory of God..." (pg 4)

So, he gave up medicine and bought three slaves and bought land on the banks of the Alabama River. He returned to the city of Saint Stephens only once to search for a wife. He died rich.  What did he do on the land?  Well, the men of the family stayed on the farm and "....made their living from growing cotton." (pg 4)

There are actually two answers to your question.  He practiced medicine initially but also grew cotton.