Johnny Fossett, 33, apologized Monday in Montgomery County court to relatives of 40-year-old Lori Jefferson, of Springfield, saying he never intended to kill her.
Authorities said Fossett called 911 in January 2012, telling a dispatcher "I choked my old lady out, tied her up and robbed her."
Defense attorney George Griffith sought leniency, saying his client has a history of mental illness and minimal intelligence.
"You are not dealing with some cold, calculated killer here," he said.
But Deputy District Attorney Thomas McGoldrick cited Florida convictions in drug, burglary and robbery cases and called Fossett "nothing but a drain on society."
Judge William Carpenter said in imposing sentence that the defendant presented "a clear and present danger to others."
The victim's mother, Emma Jean Jefferson, said her daughter's death took away a big part of a small family.
"Today is about justice for my daughter," she said. "I have endured such hurt and sadness."