David Dearlove swung 19-month-old Paul Booth by the ankles and smashed his head into a fireplace at the family's home in Haverton Hill, Stockton.
The little boy's brother, Peter, who was just three at the time of the killing, saw it happen when he crept downstairs for a drink.
He went to police in 2015 when he became incensed after seeing a picture on Facebook of his little brother sitting on Dearlove's knee.
Dearlove, who is now 71, had denied killing the toddler, telling a jury at Teesside Crown Court that the little boy had fallen out of bed.
He was sentenced on Friday to life in prison, with a minimum term of 13 years, for the murder and an additional three counts of child cruelty.
When cross examined in court Dearlove showed little emotion, telling Prosecutor Richard Wright QC that the death of the child was "probably upsetting" but that he could not remember his feelings.
He had been left alone with Paul on the night of the murder, and had travelled with Mrs Booth to the hospital with the fatally injured child before returning home.
Toddler Paul did not regain consciousness and died in hospital four hours after the attack, suffering a fractured skull.
Although bruises and burns were also found on the child's body and childcare specialists had voiced concerns, Dearlove was never prosecuted.
As the verdict was announced on Friday he did not visibly react, but gasps could be heard from the public gallery where his family sat.
His lawyer said 50 years of "blameless character" following the murder - in which Dearlove moved to London, married and had two daughters - should be taken into account when sentencing.
Dearlove had moved in with Carol Booth and her three children, whom he told the jury he liked but did not love, when he was 21.
Peter and his sister Stephanie told the jury he had been physically abusive to them as children.
After Dearlove and Carol Booth split in 1970 Peter never saw the stepfather again until facing him in court.