The decision to fire Dominique Heaggan-Brown had nothing to do with Smith's shooting
, which sparked unrest in parts of the city.
It stems from allegations of sexual assault against the officer that came to light after the fatal shooting.
An internal police investigation found Heaggan-Brown to be in violation of departmental policies, the Milwaukee Police Department said.
"Department members shall not behave in such a way that a reasonable person would expect that discredit could be brought upon the department, or that it would create the appearance of impropriety or corruptive behavior," the department said.
The alleged assault occurred in the early morning of August 15, two days after Smith's August 13 shooting.
Heaggan-Brown and the alleged victim connected on Facebook and the two men agreed to meet at a bar. They talked and drank as coverage of the Sherman Park protests aired on the television, the complaint states.
After assaulting the man at the officer's home, Heaggan-Brown took him to a hospital and told a nurse the man "began to act weird and unresponsive" at a bar, according to the complaint.
Heaggan-Brown told a hospital security officer the man was "completely out, zonked out of his gourd" after drinking, the complaint said.
Later, after seeing Heaggan-Brown in the emergency room, the man told hospital workers, "He raped me. He raped me," according to the complaint.
Heaggan-Brown was charged with one count of felony sexual assault stemming from the August incident.
Police said Heaggan-Brown turned over his phone to investigators and let them search his home. The phone turned up evidence the officer propositioned two others for sex, as well as video and photos of alleged sexual encounters, according to the complaint. The evidence led to four charges of sexual assault, prostitution and capturing an intimate representation.
Heaggan-Brown has pleaded not guilty to two misdemeanor counts. He is expected to enter a plea to three felony counts at his arraignment Friday.
Heaggan-Brown's attorney, Michael Steinle, could not be reached for comment.
Heaggan-Brown came to the spotlight after Smith's shooting. Heaggan-Brown and another officer chased Smith after a police stop. Smith and another man fled the car, and the officers followed, shooting Smith in the arm and chest when he failed to put his gun down, according to authorities.
Smith's handgun, along with 500 rounds of ammunition, were stolen during a burglary in nearby Waukesha in March, police said.
Smith died at the scene. No officers were injured.