Smith was being held in Harris County jail without bond. He appeared in court for a preliminary hearing Wednesday, though Harris County court records do not list an attorney for him.
Over the last three years, police have reviewed evidence and re-interviewed witnesses and suspects in the case, but it wasn't until Monday that officers spoke with Smith and arrested him after "he made some incriminating comments," Lt. Richard Kleczynski said.
Police wouldn't provide details of Smith's statement, or what evidence pointed to him, but "the suspect confessed to murdering both Pamela and Tashona," Kleczynski said.
Sgt. Eli Cisneros, who interviewed Smith, said he seemed "not at all remorseful" but relieved.
"He seemed to just want to put this behind him," Cisneros said. "It was obviously something great he'd been holding all this time."
Investigators said Clarence wanted to break up with Smith after learning he had another girlfriend who bore him a son one month after Tashona was born. Smith became upset and strangled Clarence and then suffocated Tashona, police said.
Smith hadn't been questioned since the time of the murders. He was one of two possible suspects in the deaths, but Senior Police Officer Connie Park said DNA evidence collected at the time couldn't point to one person.
"In this case, we could only make the (arrest) by a confession," Park said.
Smith has since worked as a stock person in a Woodlands grocery store.
Clarence's sister, Jo Ann Petitt, said she was stunned when officers called her Tuesday and told her about the arrest. She said her large family never gave up hope that police would find the killer.
"I can remember like it was yesterday," said Petitt, who found her infant niece under a pile of blankets in a bed after another relative found Clarence's body and a neighbor called 911. "Not a day goes by I don't think about her."
The family has not seen Smith since the murders but always suspected him, she said.
"All of us said from Day 1 it had to be the baby's daddy," Petitt said. "Who would kill a one-month old child? It had to be the daddy."