Attorney General Henry McMaster said the governor's use of pricey airline tickets, travel to personal and political events on state aircraft and questionable campaign reimbursements had not risen to a criminal level.
"Those punishments are sufficient," McMaster said Monday, referring to Sanford's civil ethics charges and censure by state lawmakers. "The time has come for our state to put this controversy behind us and move on."
McMaster for months had been reviewing a probe by the State Ethics Commission that started after a series of Associated Press investigations were published about Sanford's use of state, private and commercial planes. The state probe led to 37 charges and $74,000 in civil fines that Sanford paid in March.
The scrutiny started when Sanford disappeared last June, returning after five days to admit to an extramarital affair with an Argentine woman he later called his "soul mate."
The Republican governor's second and final term ends in January. A judge approved Sanford's divorce from his wife in March.
State lawmakers used the ethics investigation as the basis for a formal rebuke of Sanford after deciding the charges didn't merit impeachment. Lawmakers kept the door open to that if McMaster came forward with criminal accusations.
McMaster is one of four Republicans seeking to replace Sanford. The prosecutor had faced recent criticism from his three GOP opponents for the time it was taking him to release the results of his investigation.
In an interview with the AP on Thursday, McMaster shed a glimmer of light on the work his office had been doing and why it's taken five months.
"There are facts that needed to be determined outside of the scope of what we received from the Ethics Commission," McMaster said. "There was a lot of analysis that needs to be done."
A spokesman for Sanford said the governor would issue a statement later Monday.
McMaster's critics have said he should have appointed an independent prosecutor to consider the charges. But McMaster said last week his office can prosecute fairly even in cases of people it knows.
The Republican primary in the governor's race is June 8.