The execution was stayed Tuesday by a federal appeals court, and the state appealed. Gov. Jay Nixon said in a news release the high court had allowed the execution to proceed and he had turned down a request for clemency, KDSK-TV reported.
The execution was originally scheduled for 12:01 a.m. CT with the warrant valid until 11:59 p.m. CT.
"The jury that convicted Nicklasson determined that he deserved the most severe punishment under Missouri law, and my decision on clemency upholds the jury's action," Nixon said. "Both the conviction and the death sentence of Allen Nicklasson have held up under extensive judicial review by the state and federal courts."
Nicklasson and two other men were convicted of killing Richard Drummond, a businessman who offered them help when their vehicle broke down on I-70 in 1994.
Dennis Skillicorn was put to death in 2009 for the killing. Tim DeGraffenreid, who was 17 in 1994, is in prison.
Nicklasson has also been sentenced to life in prison in Arizona, where he and Skillicorn killed a couple who offered to help them after they got stuck in the desert.
Mike O'Connell, a Public Safety spokesman, said Wednesday afternoon the prison in Bonne Terre remained on alert, waiting for information from the high court or the state attorney general, Missourinet.com reported.