The Supreme Court of Appeal in Bloemfontein more than doubled his original sentence of six years after the state appealed that it was unduly lenient.
Pistorius became the first amputee runner to compete at the Olympics and was even known as the "fastest man on no legs" and the "Blade Runner".
Johnson said the High Court did not list the substantial and compelling factors for deviating from the 15-year sentence and that Pistorius had not shown remorse for the murder.
A trial judge had previously said that Pistorius acted negligently when he shot Steenkamp four times, but did not intend to kill her.
"They have sent all their love to the Ings family because they feel that although justice was serviced for Jayde, they know the Ings family will still miss Jayde every day".
The conviction was later overturned and a murder conviction was set in place in 2015.
The prosecutors appealed again, claiming the six-year sentence was "shockingly low".More news: Greensboro church gives Thanksgiving meals to first response crews
More news: ASU considers revoking award given to Charlie Rose
More news: Bill Clinton accused of sexual assault . . . again
The ruling added that Pistorius "displays a lack of remorse and does not appreciate the gravity of his actions".
The court said Pistorius should have been sentenced to 15 years, but the SCA took into account the time he had already served.
The new sentence was backdated to start on the day he began his murder sentence, on July 6 previous year. He received a six-year sentence for this original conviction.
"Leave to appeal should really not be granted", he said. He was reportedly not in court on Friday to hear the decision on his sentence.
Pistorius' lawyers have one avenue left open to them if they want to challenge the new sentence, and that is to appeal to the Constitutional Court, the highest court in South Africa.
This is the third time Pistorius has faced a different sentence in the case.
Pistorius was jailed in July 2016 after being found guilty on appeal of murdering Ms Steenkamp on Valentine's Day 2013.