Horn was announced as the world's newest welterweight champion as a storming Brisbane crowd screamed in elation.
Rarely do blockbuster fights ending via decision escape controversy, but the fallout from Horn's shock victory was something else entirely.
Post-fight stats revealed Pacquiao almost doubled Horn's number of punches landed in a 182-92 split.
US sports media exploded at the judges' 115-113 x 2, 117-111 result immediately.
Boxing expert Teddy Atlas was fuming, claiming the 29-year-old was gifted the decision for how hard he worked as opposed to how many clean punches he threw. The 60-year-old exploded on-air after Horn's arm was raised, slamming the judges in an incredulous tirade.
"They gave a win, a huge win, to the local kid for trying hard," he yelled. "You're not supposed to get it for trying hard. You're supposed to get it for winning and I thought Pacquiao won the fight if you go by the real rules of who lands the cleaner punches."
ESPN reporter T.J. Quinn was equally outraged, claiming Horn won because the fight was held on his home turf. "No way in hell Horn won unanimously," he tweeted. "A 117-111 card? Ridiculous. This is a hometown hit job. Manny looked old, but mostly in control."
Famous sports columnist Skip Bayless also questioned if the result had to do with the fight taking place in Brisbane.
FOX Sports reporter Dieter Kurtenbach was even more critical, labelling the fight "rigged". Green Bay Packers quarterback Aaron Rodgers took an almighty swipe at boxing, calling the sport a "joke".
Surprisingly, Pacquiao had no qualms about the result immediately after the decision, accepting the call as medical staff rushed to work on his cuts.
"That's the decision of the judges, I respect that," he said.
When asked if he'd be returning later in the year for a rematch, the former world champion replied: "Absolutely, no problem."
Horn called out out boxing immortal Floyd Mayweather in a bizarre post-fight sledge, asking the 40-year-old - who is preparing to fight UFC lightweight champion Conor McGregor in August - if he wanted "the walking stick or gloves" before telling him to "come have a real fight".