A wake was held at Smith, Seaman & Quackenbush Funeral Home in Monroe.
His funeral will be held Friday 11 a.m. at the Church of the Sacred Heart, located at 26 Still Road in Monroe.
More than 25,000 people are expected to attend the services, prompting the Monroe-Woodbury Central School district to close schools Friday.
Officer Mulkeen was a graduate of Monroe-Woodbury High School.
"We are deeply saddened by the tragic loss of M-W alumnus NYPD Officer Brian Mulkeen," Superintendent Elsie Rodriguez said in a statement. "Our heartfelt condolences go out to his family and friends, especially his mother, Camille, who served on the Monroe-Woodbury Central School District Board Education. The entire Monroe-Woodbury community mourns the senseless death of this young man who selflessly dedicated his life to the service and protection of others."
Many officers in uniform came to the wake to pay their respects to Mulkeen. Also attending were Mayor Bill de Blasio, NYPD Commissioner James O'Neill, and Bronx Borough President Ruben Diaz, Jr.
"It takes heroes like Mulkeen to get those few individuals off the streets so that people can have a better quality of life so that our kids can have an opportunity and that's what he was doing that night," said Diaz.
Mulkeen had left a job in finance to pursue a career as a police officer. He served nearly seven years on the force.
He lived in Yorktown Heights with his girlfriend, who is also a police officer in the 44th Precinct in the Bronx.
Those who worked with Mulkeen say he was devoted to his work as an officer in the Bronx anti-crime unit. He had 270 arrests, the majority for removing illegal guns from the streets.
"He was the best of the best," said Commanding Officer Jeffrey Helig. "He was a gentle giant and I mean that. When it was time to be on the street to be the police he was. When it was time to be compassionate and have a heart he did."
The Tuxedo Police Department, where Mulkeen had worked previously, also came in solidarity.
"He was very driven," said Tuxedo Police Officer Girard Shiloh. "He knew what he wanted to do. He wanted to be in the NYPD."
The NYPD announced Monday that Mulkeen was struck by friendly fire.
Police Commissioner James O'Neill said Mulkeen was hit twice by police bullets while wrestling with an armed man during the confrontation near the Edenwald Houses in the Bronx.
Related: Who is NYPD Officer Brian Mulkeen?
The suspect, identified as Antonio Williams, of Binghamton, has various prior arrests according to police and was also killed.
"There is absolutely no worse moment on our job than this," Chief of Department Terence Monahan said. "As we stand here this morning, a young man with a bright future who courageously patrolled some of New York City's toughest streets has tragically lost his life."
As the investigation continues into the officer's death, the NYPD released video from Thursday night of a shootout in the Edenwald Houses, two nights before Mulkeen was killed.
Increased tensions between rival gang factions in the city run housing complex, mostly over sales of $5 marijuana bags, led to the gunfire depicted in the video released by the NYPD. Up to 30 shots were fired just before 7:45 p.m. No one was struck by the gunfire, and the NYPD is looking for nine suspects.
The next night, the Bronx anti-crime task force was sent in to find the guns, and Mulkeen caught a 27-year-old man carrying a Smith & Wesson .40-caliber revolver. The suspect was charged with criminal possession of a loaded firearm and unlawful possession of marijuana.
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