Betty Jarvis and Wanda Carney had been found guilty of conspiracy and obstruction, after allegedly interfering in the 2005 probe of a drug informant's killing.
Jarvis' nephew, Walter Harmon, was among those charged in a plot by a Mingo County drug ring to murder Carla Collins. Carney worked for Harmon's defense lawyer.
Prosecutors alleged the pair hampered State Police efforts to interview two witnesses. Jarvis also removed items from the apartment of one of two people later convicted in the murder.
Friday's ruling concludes the duo's actions did not amount to crimes. It is unsigned, meaning it sets no legal precedent.
Defense lawyers argued that Jarvis was aiding Carney as she tracked down leads for Harmon's defense. While interviewing the witnesses, the pair had relayed rumors that police officers had been romantically involved with Collins and were covering up her murder.
Carney also said she had permission from the apartment's landlord - later convicted of an abetting role in the murder plot - to enter the residence of key suspect Valerie Friend.
Convicted in 2006, Carney and Jarvis have already spent 120 days under electronic monitoring and performed 200 hours of community service. Friday's ruling ends their three-year terms of probation.
Carney continues to work as a defense investigator, and remains an activist focused on county government and school board issues. She said the convictions placed a cloud over such efforts, with some officials invoking the criminal case in an effort to intimidate her.
''I'm elated that my name is cleared,'' she said Friday. ''It's been a very, very trying time.''
Carney wants the State Police to investigate the pursuit of charges against her and Jarvis. Mingo County Prosecuting Attorney Michael Sparks expressed disappointment in the reversal but stood by the case.
''I believe the prosecution was supported by the evidence, and was the right thing to do at the time,'' Sparks said Friday. ''And a Mingo County jury agreed.''