Richmond Phillips, 40, was convicted Thursday of two counts of first-degree murder by a jury in Prince George's County in Maryland, The Washington Post reported.
Wynetta Wright, 20, was shot outside a community center in Hillcrest Heights in May 2011. Her daughter, Jaylin Wright, 11 months old, died in her sport utility vehicle, which was parked at a nearby apartment complex.
Prosecutors said Phillips, who is married, did not want to pay child support. He was scheduled to take a DNA test to determine paternity the day Wright was killed.
Wyvette Wright said after the verdict Phillips would be a "perfect candidate" for the death penalty for killing her daughter and grandchild.
"I am just so happy that he was found guilty on every count because he is a liar and a killer," she said outside court.
Phillips' wife and another girlfriend testified for the prosecution. Jurors also saw surveillance video that showed Phillips and Wright at the community center not long before she was killed.
Defense attorneys say they plan to appeal. They argue prosecutors had no evidence Phillips actually killed Wright and said he lied to investigators because he wanted to conceal his affair with her.
Prosecutors say they will seek the maximum penalty, life without parole, when Phillips is sentenced.