Lankford had been sentenced to death by an Idaho County judge in 1984 for the beating deaths of Texas Marine Captain Robert Bravence and his wife Cheryl at a remote campsite outside of Grangeville.
Mark's brother, Bryan Lankford, was also convicted and both brothers blamed the other for the crime.
Lankford appealed his conviction, and his case was overturned by the 9th U.S. Circuit Court of Appeals. He faced a retrial in 2008 and again found guilty of both murders.
The district court sentenced Lankford to two consecutive fixed life sentences after which Lankford again appealed.
The Idaho Supreme Court noted that prosecutors failed to disclose additional promises made to a key state witness before Lankford's second trial.
The court found Lankford's constitutional rights to a fair trial were violated and hence vacated his conviction.