The driver of the truck was identified by police sources as Charles Loughran, 39, an 18-year police veteran assigned to the 15th Police District, with headquarters at Harbison Avenue and Levick Street in Northeast Philadelphia.
Loughran suffered head injuries in the accident and was listed in serious condition early today at Methodist Hospital.
McCarthy's horse, a 7-year-old chestnut gelding named Skipper, had to be destroyed at the scene, police said.
McCarthy had just finished working crowd control outside the Spectrum, where the Grateful Dead rock group was playing, and was heading back to the horse barn with two other mounted officers when he was struck, police said.
The truck, which was heavily damaged in the accident, continued for about 100 feet south on Broad Street before coming to a halt, according to police.
As emergency vehicles sped away with McCarthy and the driver of the pickup, the horse lay on South Broad Street near the median. The saddle, apparently torn off in the accident, was lying about 50 feet away.
A police officer destroyed the animals, and its body was hauled up a ramp by cable into the back of a truck, which drove away about 11:30 p.m.
McCarthy was assigned to the mounted unit and worked out of the 25th Police District, with headquarters at Front and Westmoreland Streets, according to Detective Gerald Whartenby, a public-information officer. McCarthy had worked in the Sixth Police District and in the Juvenile Aid Division before transferring to the mounted unit in 1985.
McCarthy lived in the 6200 block of North Third Street in the East Oak Lane section. He was married with four children: a daughter 8, twin daughters 5 and a month-old son. He had earned five commendations during his 10 years on the police force.
Whartenby said there was a similar accident involving a mounted officer and his horse about 10 years ago in the same general area, but "this is the first time that we are aware of that an officer has died."
The mounted unit was dissolved in the early 1950s but was reactivated in the early 1970s by Mayor Frank L. Rizzo. Mounted officers are used extensively for crowd control.
Mayor Goode, Managing Director James S. White and high-ranking police officials, including Police Commissioner Kevin M. Tucker, went to Methodist Hospital last night and spoke with members of McCarthy's family.
"It's very tragic, very sad," Goode said as he left the hospital shortly after 1 a.m. "My heart goes out to the family. It's a tremendous loss for them."
Police said their investigation was continuing.
Whartenby said police were trying to determine the speed of the truck at the time of the accident. Investigators were not immediately able to question the driver of the truck because of his injuries, Whartenby said.