The suspect, Taweewong Tongyuen, 55, a former Phanthong TAO president, was accused of premeditated murder of Ms Veerayapha Ngamwilai, 36, his ex-wife
Taweewong however denied the charge upon arrest, although admitted of divorcing her two years ago.
But after intense questioning he later confessed to masterminding the shooting of her on March 14 in Klong Luang district of Pathumthani.
The gunman who he identified as Muek is still at large.
Jealousy is the motive of the shooting, police said.
Veeraya was shot dead inside her car while she parked on the Wang Noi-Thanyaburi road in front of local administration office.
A CCTV video camera of the local administration office captured a suspected pickup truck coming behind and two men were seen walking to her car. One man when inside and another standing behind.
Shortly later her car was seen running backward and falling into the roadside bushes and the two men then sped in the pickup truck.
From the video footages retrieved from the scene and a toll gate, the licenses plate of a suspected car was also captured.
They led the Police investigators to trace to the former TAO president.
Taweewong later confessed that although he had divorced her, he still kept in touch and financially supported her.
But he said he was angry after finding that she cheated him.
Police are still hunting for the hired gunman.