Carmela dela Rosa, of Fairfax, claimed she did not know right from wrong when she threw Angelyn Ogdoc off the 45-foot-high skywalk at Tysons Corner Center.
Prosecutors said dela Rosa was motivated by anger at her son-in-law for getting her daughter pregnant out of wedlock, prosecutors said.
Jurors saw a videotaped confession in which dela Rosa told detectives that she saw Angelyn as a way to get back at the son-in-law, James Ogdoc.
Defense witnesses testified that dela Rosa was diagnosed with severe depression and tried to kill herself on several occasions in the months before the incident.
One of Dela Rosa's neighbors, Russell Jackson, described them as "a happy family" and said Dela Rosa lived with her husband and a son, but that their daughter and granddaughter were frequently at the home.
"She was a sweet, happy baby," he said, adding that Dela Rosa and her grandchild could often be seen in the window of the home, waving to passers-by.
Dela Rosa faces 20 years to life in prison.