The suit, filed in Common Pleas Court, also names Brennan and Msgr. William J. Lynn, the former church official responsible for investigating allegations of misconduct by priests.
Lynn, 62, is the former secretary for clergy who was convicted of child endangerment last year and is serving three to six years in prison. According to trial testimony, Lynn took part in transferring Brennan from parish to parish while he repeatedly molested preteen boys.
Archdiocesan spokesman Kevin A. Gavin said church officials had no comment on the lawsuit.
Marci A. Hamilton, who along with cocounsel Daniel F. Monahan filed the suit on behalf of Sean McIlmail's estate, said it was the third claim brought against the Philadelphia church involving allegations about Brennan, and the second wrongful-death suit here involving an accuser of a pedophile priest. Other wrongful-death suits have been filed by accusers' families in clergy-abuse cases as far away as Missouri and California.
Brennan, now 75 and living in Perryville, Md., figured prominently in two Philadelphia grand jury reports on molesting of minors by archdiocesan priests. He was not criminally charged, however, because the reported alleged incidents happened too long ago to prosecute.
After the 2005 grand jury report, church officials ordered Brennan to relinquish all priestly duties, and he retired.
His situation began to change after last year's highly publicized trial of Lynn, the first Catholic Church official convicted for a supervisory role in covering up the conduct of pedophile priests.
McIlmail said Sean did not tell her until last fall, after undergoing psychotherapy, of the molesting he had suffered. She said his disclosure helped make sense of his years of emotional difficulties and drug abuse.
Encouraged by Lynn's conviction and sentence, Sean McIlmail contacted church and civil authorities in January to say he wanted to press charges.
In September, the District Attorney's Office brought rape and other charges against Brennan for allegedly molesting Sean from ages 11 to 14, beginning in 1998, when the boy was a member of the "altar guild" at Resurrection of Our Lord parish school in Rhawnhurst.
Brennan has never commented on the allegations against him by McIlmail or others cited in grand jury reports. After McIlmail's Oct. 13 death, Brennan's lawyer, Trevan Borum, said his client was disappointed at not having a chance to clear his name in court.
McIlmail's death ended the criminal case against Brennan - prosecutors had lost their witness. But Deborah McIlmail and her lawyers said they hoped Sean's courage in coming forward would help others do the same.
At Wednesday's news conference, McIlmail, joined by her son Michael, 27, and daughter Kaitlyn, 24, said she knew Sean was committed to following through with the Brennan prosecution to prevent aberrant clergy from abusing other children.
McIlmail noted that church officials had first received complaints about Brennan's behavior toward boys in 1988 - when "Sean was just 2 years old," she said.