Mikail Markhasev, a 19-year-old Ukrainian immigrant, also was convicted of attempting to rob Cosby and using a firearm in the commission of attempted robbery.
The six-man, six-woman jury's finding on all counts automatically mandates a life prison term with no possibility of parole.
"The Cosby family is satisfied with the judicial process that has led to this conviction," Cosby spokesman David Brokaw said. "They have no comment on the sentencing."
In the courtroom, members of the Cosby family began to weep and hug each other as the verdicts were announced. Although Bill Cosby and his wife Camille were not present, two of their daughters Erika and Erinn sat with several other relatives in the front row.
Also in the family group was Phil Caputo, the man who played basketball with Ennis Cosby hours before he was killed. Caputo had tears in his eyes as he heard the word "guilty."
The family of Markhasev never made it to the courtroom in time to hear the verdicts. Markhasev and his lawyer stood stonefaced, staring at the jury as the verdicts were read.
Ennis Cosby, 27, a graduate student at Columbia University who was vacationing in California, was fatally shot Jan. 16, 1997, while changing a flat tire on a dark road. He was Bill Cosby's only son.
Markhasev, who has a history of gang affiliations and a previous brush with the law, was arrested nearly two months later.
The jury, which heard testimony over two weeks, spent less than six hours talking over the case before accepting the prosecution's argument that Markhasev had been convicted by his own words.
The trial was expected to take more than a month, but the prosecution and defense presented very brief cases.
The case went to the jury Monday after Bill Cosby made his first court appearance on the last day of arguments. He had stayed away from the trial until then, saying he wanted to preserve the dignity of the proceeding.
The defense suggested that police arrested the wrong man and pointed the finger at Eli Zakaria, one of two other people with Markhasev at the time. The defense also keyed on reward money a prosecution witness was seeking from a tabloid.
The prosecution presented only one eyewitness from the killing scene, Stephanie Crane. She said she had a date with Ennis Cosby, but could not identify Markhasev.
Prosecutors, however, produced jailhouse letters allegedly written by Markhasev. It includes the statement: "The crime happened in Bel-Air. A robbery gone bad." It concludes: "I went to rob a connection and obviously found something else." It closes with a picture of a happy face and is signed "Peewee."
©1998 CBS Worldwide Corp. All rights reserved. This material may not be published, broadcast, rewritten, or redistributed. The Associated Press and Reuters contributed to this report