Rodney Sanders (pictured below) was arrested on Friday and charged with murder over the deaths of Sister Margaret Held and Sister Paula Merrill (pictured above), both 68.
It's claimed, while admitting to the crime, the 46-year-old gave no reason for their deaths.
The sheriff said police work and tips from the community led them to Sanders.
It's reported Sanders was convicted last year of a driving under the influence offence but was released from prison and is currently on probation.
He was also convicted of armed robbery in 1986 and served six years.
It's unclear whether the nuns were aware of Sanders before they were killed.
Rev Greg Plata from t St Thomas Catholic Church in Lexington where the women led bible study for years said he did not think people at the church knew Sanders.
David Merrill, Sister Paula's nephew, said the family was "thankful" Sanders was off the streets.
He said forgiveness was something his aunt would want for whoever killed her, but he was not sure if he was capable of completely forgiving.
The women's bodies were discovered on Thursday after they failed to arrive for work in Lexington, about 10 miles from where they lived.
Their stolen car was found abandoned a mile from their home, and there were signs of a break-in, but police have not disclosed a motive.