Jessica Delrocco told ABC's "Good Morning America" on Wednesday that Clark dictated what clothes she could wear, where she could go and what friends she could have.
He would also get very angry and "physical" with her, to the point where she was frightened, she said, but declined to elaborate on the alleged confrontations.
Clark stands accused of strangling Le, a petite 24-year-old Yale graduate student, and stuffing her in a Yale lab wall.
Her body was discovered last Sunday, five days after she went missing. She was supposed to be married on Sunday in Long Island.
One of Clark's lawyers, public defender Beth Merkin, declined to comment on Delrocco's assertions.
Le and Clark came into contact with each other through their work at the Yale lab. Her team experimented on mice as part of research into enzymes that could have implications for the treatment of cancer, diabetes and muscular dystrophy.
Clark was responsible for caring for the mice, cleaning floors and cages. An official told the AP that Clark's Yale co-workers told police he was a "control freak," fastidious and territorial about the mice whose cages he cleaned, even with scientists and doctoral students at the Ivy League school.
Clark, who began working at the lab in 2004, attended high school in Branford, a small middle-class suburb of New Haven.
In 2003, when he was a senior at Branford High, police reportedly warned him to stay away from a girlfriend because he upset her so much when she tried to break up with him. The New Haven Independent, obtained a copy of the police report and reported the incident last week.
It is unknown if Delrocco is the girlfriend described in the report, and police refused to give a copy of the report to The Associated Press, citing the investigation into Le's murder.
Delrocco didn't immediately return a message the AP left Wednesday with a relative.
Le's funeral is set for Saturday in El Dorado Hills, Calif., near her hometown of Placerville.
She was also to be remembered Wednesday at a memorial service at her fiance's synagogue in New York.