Benjamin Roach, 24, from Plymouth, violently shook Roxie-Jo Hopkins-Roach on September 2010, leaving her fighting for her life.
Roach denied her manslaughter, but was convicted unanimously by the jury at Plymouth Crown Court.
The hearing was told that Roach may have shaken Roxie-Jo before throwing her down on to a bed or sofa, leaving her with injuries similar to what might be seen as a result of a road crash.
The jury heard that if she had survived the assault, she would have been left blind.
Ms Hopkins had left Roxie-Jo with Roach while she went out to buy her some medicine. Roach told police that Roxie-Jo was 'screaming' and 'crying heavily' and that he had picked her up to comfort her when she had suddenly become floppy.
He later told police a different story, saying he had dropped her around four feet on to a carpeted floor, and had shaken her to rouse her when she lost consciousness.
Roxie-Jo was taken to hospital on the day of the incident, but died in the high-dependency unit the following day.
Kelly Hopkins said after the trial that her daughter 'can now rest in peace and justice has been served'.
"Losing my daughter, my parents' granddaughter and niece to my brother and sister will always remain the hardest thing that has happened to us," she told the BBC.
"Although now all I have is memories, these are memories I will cherish throughout the rest of my life. The justice I have fought for, for finding the truth about what happened to Roxie and knowing she is now in a better place, has been all I've wanted and now I can finally say I have achieved it."
Detective Inspector Costa Nassaris, from Devon and Cornwall Police, said that justice had been done for the family.
"I am glad that justice has finally been done for baby Roxie and her mother, Kelly," he said. "She has shown great courage and determination in her desire to get answers for herself and get justice for her daughter."
Roach will be sentenced today.
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