Justify is a chestnut colt with a large white blaze on his forehead. He was bred in Kentucky by Canadian John D. Gunther, who runs Glennwood Farm with his daughter Tanya. Although the Gunthers have a small breeding operation, their 2015 foals also included
Without Parole, a Group I winner at Royal Ascot, and Vino Rosso, a graded stakes winner who raced against Justify in both the Kentucky Derby and Belmont Stakes. Gunther also bred Justify's dam, Stage Magic, a stakes-placed daughter of Ghostzapper.
Justify was part of the second-last crop of Scat Daddy, who won the Champagne Stakes and Florida Derby. Although Scat Daddy ran in America on the dirt, as a stallion he first made his reputation in Europe as a turf sire, with Group 1 winners including Lady Aurelia and Caravaggio His stud fee when he was bred to Stage Magic in 2014 was only $35,000, though it was scheduled to increase to $100,000 in 2016. Scat Daddy died prematurely at the age of eleven in December 2015.
Justify was sold at the 2016 Keeneland September Yearling Sales for $500,000 to a recently formed partnership between WinStar Farm (60% ownership),
China Horse Club (25%) and SF Racing (15%), the latter run by employees of George Soros's investment firm. David Hanley, general manager of WinStar, said that Justify stood out at the sales. "He was a beautiful yearling – had great balance and leg, great shoulder, and lovely length and neck, and power," he said. "For a big horse, he was very light on his feet, with good action and good energy." In early March 2018, SF Racing sold a share to Starlight Racing, then Sol Kumin's Head of Plains partners also bought a share in late March.
Justify grew into a large horse, standing 16.3 hands (67 inches, 170 cm) at the withers and weighing 1,380 pounds (630 kg). He was initially sent to trainer Rodolphe Brisset at Keeneland but pulled a muscle and was given time to recover and grow into his frame. In January 2018, he was sent to Santa Anita to be trained by Bob Baffert.