Stephen Carl Smith had agreed to babysit while Luke Taylor and his partner Sophie Reed went night fishing, Swansea Crown Court heard.
Smith, 28, of Haverfordwest, had denied causing grievous bodily harm with intent but a jury found him guilty.
Judge Peter Heywood said Smith had lost his temper with the baby, Bayleigh-Lee.
The couple had returned home from Milford Haven pier, Pembrokeshire, after Smith sent Mr Taylor a text message saying his daughter was having difficulties breathing, the court heard.
When they arrived, Bayleigh-Lee was sitting in a chair and appeared to be asleep.
But the court heard that while Miss Reed was in the kitchen, the baby let out “a piercing scream” and when she picked her up found her limp and lifeless.
Doctors found there had been bleeding to the brain.
Bayleigh-Lee suffered “catastrophic and life-changing” injuries in the incident on 18 August 2016, the jury was told.
Smith, of Hywel Road, claimed Bayleigh-Lee had suddenly fallen ill. He had maintained he was innocent and refused to apologise.
The jury heard the two men had been friends and Smith would sometimes help Mr Taylor, a self-employed delivery driver.
“She will need medical care for the rest of her life. Something went disastrously wrong that evening,” said Judge Heywood, sentencing Smith.
John Hipkin, counsel for Smith, said all he could offer on his client’s behalf was that he had not offended in a similar way before.