The image was shared widely and led to thousands of tips, but the child's identity remained a mystery for months.
Monday's guilty verdict comes two years and one day after the little girl's body was discovered washed ashore on Deer Island in Winthrop. It found him guilty of second-degree murder.
McCarthy will be sentenced Wednesday; he faces life in prison with the option of parole after 15 years.
Even now, prosecutors can't say for sure what drove McCarthy to kill the child, nor does the law require them to prove motive.
Jonathan Shapiro, McCarthy's attorney, said they will appeal the decision.
"I think it was very hard to determine who actually killed Bella".
"An innocent man is going to prison for the rest of his life for a crime that she (Bond) committed", Shapiro said.
McCarthy was the boyfriend of Bella's mother, Rachelle Bond.
"I could not be more thankful for the work done by the Suffolk District Attorney's Office and our State Police Detective Unit for Suffolk County in holding Bella Bond's killer accountable", Massachusetts State Police Colonel Richard McKeon said in a statement.
The trial included testimony about the girl's short and tragic life.
Assistant District Attorney Deakin emphasized to the jury that they must not judge McCarthy, Bond and the other individuals involved in the case as a group of addicts. A friend testified that McCarthy would lock the girl in a closet to discipline her.
Michael Sprinsky, a former friend of McCarthy's who helped crack the case for police, was also emotional remembering the 2-year-old.
He blamed Bella's death on addiction, noting that "By the time of her death every adult in her life was distracted from her care and well-being by drug addiction".
Bond testified that on the night Bella died, she walked into the girl's bedroom and saw McCarthy leaning over her and punching her.
"I do see her as a victim in this case", Bassil said of Rachelle Bond. "Can't stop puking. You killed Bella, bro", he texted to McCarthy. As part of a deal with prosecutors, she testified during the trial and will be sentenced to probation.
FILE - In this May 22, 2017, file photo, Michael McCarthy watches as jury selection begins for his murder trial in Suffolk Superior Court in Boston.
"Rachelle Bond's testimony was clearly valuable and credible because it was corroborated by forensic evidence that she did not know we would obtain", Conley said, citing cell phone records that corroborated her testimony about the couple's whereabouts in the days following her death.
Michael McCarthy was found guilty of second-degree murder in the death of Bella Bond, the 2-year-old who became known as "Baby Doe" after her unidentified body was found in a plastic bag near the Boston shoreline in 2015, according to multiple reports.
Bond and her boyfriend, Michael McCarthy, were arrested in September 2015 in the killing earlier that year of Bella Bond.
Rachelle Bond in court June 7, 2017.