Monjack's death comes just five months after Murphy's death. He was found dead in the Hollywood Hills home the couple shared.
The preliminary cause of the death of the British screenwriter is natural causes, police spokesman Sgt. Louie Lozano told The Associated Press.
"We concluded there no signs of foul play or any criminal activity involved," said Sgt. Alex Ortiz, another police spokesman.
It was Brittany Murphy's mother who found her son-in-law's body Sunday at 9:40 p.m. He was unresponsive in the master bedroom. Paramedics tried to revive him but were unable to do so.
According to family spokesperson Roger Neal, Murphy's mother and Monjack were working hard to expand the Brittany Murphy Foundation, an organization dedicated to helping underprivileged children learn music and art.
Neal said he saw Monjack Saturday night and had spent hours with him planning a gala for that foundation which was slated for this coming fall. He described Monjack as "a guy full of life" at a Monday press conference.
"I didn't look at him and say, 'Man, there's a sick man.' I looked at him and said, 'There's a guy who is focused and who loved his wife, who loved his mother-in-law as a son would love his mom and just very excited about the future," said Neal.
Neal added that Monjack had elected to have heart surgery after that gala.
"The family is devastated over his passing," said Ed Winter of the coroner's office.
Monjack's death is under investigation by the coroner's office and the LAPD. There were prescription drugs in the home, but the coroner did not release what type of drugs they were. Authorities said there were no illegal drugs found in the home - only medication prescribed to Monjack.
The coroner said Monjack's exact cause of death will not be confirmed until toxicology tests are complete. Monjack's autopsy is scheduled for Tuesday.
Monjack's death comes five months after the death of his 32-year-old wife, who died on Dec. 20 after going into cardiac arrest. Her death was ruled accidental.
Monjack said in an interview with CNN that Murphy was his best friend and soul mate. The two married in 2007.
Investigators said Murphy's death was likely preventable. She died of pneumonia, complicated by iron deficiency, anemia and multiple drug intoxication. An autopsy found no evidence that Murphy abused drugs. Her mother and Monjack had said she had flu-like symptoms in the days before her death.
Monjack is credited as producer and co-writer of the 2001 film "Two Days, Nine Lives" and executive producer of the 2006 "Factory Girl."
The Associated Press contributed to this story.