The victim has been identified as Tammy Blount. The suspect is her son, 27-year-old Malachi White.
"He has some mental health issues and our understanding at this point is that he was not taking his medication," said Philadelphia police Capt. John Ryan. "He's going to be charged with murder and related offenses."
Investigators believe Blount died on Jan. 22 from stab wounds and blunt force trauma.
Officers discovered her body Wednesday morning while responding to a welfare check. They say family members hadn't heard from her.
There was shock and disbelief for residents of the apartment complex where the crime occurred. Many had no clue what happened inside their building.
"I was wondering, but when I asked the concierge person they thought she was sick, so I don't know," said building resident Britnee Wooden.
Police son murdered mother lived with body for 2 days
"It is messed up," said Kayan Fuller, who lives in the building. "There is nothing else to say about it. It is just a messed up situation."
The apartment management released a statement that reads in part: "Our management team, concierge and door staff observed nothing that would have indicated any problems within the family or propensity for this family tragedy."