Duckett was also convicted of two weapons offenses.
The jury deliberated for more than three days before returning a verdict just before 4 p.m.
"We are very pleased for the family and we appreciate the attention the jury gave to this case," Assistant Prosecutor Gina Iosim.
Duckett came to the Montclair YMCA on June 26, 2008. He walked to an observation area, where he got into a verbal altercation with his ex-girlfriend. While she watched their 4-year?old son swim, Duckett pulled out a gun and shot her six times at close range. Their 11-year-old daughter was seated near her mother at the time of the murder.
Following the return of the guilty verdict, the judge revoked Duckett's bail and scheduled sentencing for Feb. 1, 2011.
"While we believe justice was done in this case, this remains a very sad case of a mother who was senselessly killed in the prime of her life," Acting Prosecutor Robert Laurino said.