The court said Thursday that Carlos Cummings did not invoke his right to remain silent when he told police during an interrogation to "take me to my cell."
A state appeals court last year upheld Cummings' conviction, saying his comment about wanting to be put in a cell was unclear.
The Supreme Court agreed. It also ruled that Cummings' 24-year prison sentence was not unduly harsh.
Cummings was convicted of conspiring with his girlfriend, Carla Glodowski, of hiring Linda Dietze to kill Carla Glodowski's husband, James.
Dietze shot James Glodowski in a Stevens Point park in 2008. The man survived but lost an eye.