Oliver Fallon, 40, stole, tore and mutilated 288 records from the Scottish Catholic Archives in Edinburgh. The hard-up academic was caught after a Scottish institution he tried to sell stolen documents to recognised them.
A sheriff today ordered the Sanskrit expert to pay the archive 16,000 in compensation.
But the graduate of London College avoided a custodial term and will instead have to carry out 300 hours of unpaid work.
Fallon sold some of the documents but Catholic Heritage say the stolen and damaged files were worth a total of more than 26,000.
Fallon, who has been released from jail in England for similar crimes, told archivists he was a postgraduate student from the University of London to con his way into the high security library in Drummond Place.
Applicants who want to enter the reading room must have an interview with staff and are only allowed a pencil and notebook in with them.
But once in, Fallon set to work selecting valuable documents dating back to 1177, pocketing some and tearing others before sticking them in his notebook to sell later. He sold some of the documents for 5000 because he was having cash problems.
Librarians at the archives didn't notice the theft at first but when English police got in touch over Fallon, an archivist remembered his name and checked the databases he had accessed.
Staff were horrified to find pages torn from records, while others were missing altogether.
In all, almost 300 files had been tampered with in some way during his week-long visit to the archives in July 2006.
Fiscal depute John Cook told the court: "The archives date back to 1177 and include documents of the administration, finance and legal matters relating to the Catholic Church in Scotland. They are very rare and valuable.
"The accused ripped and tore pieces from documents and placed them in his notebook before leaving with them. Some 288 of the files examined by the accused were missing or mutilated."
Once he was caught, he sent a letter apologising and returned 14,325 worth of records.
But the damaged documents needed almost 5000 worth of repairs while 132 documents were still missing, worth 12,000.
His defence solicitor, John Mulholland, said: "He is obviously a man who is held in high regard in both his personal and professional life and this represents for him a cataclysmic fall from grace."