The younger, fresher fighter of the two, Magdaleno overcame a cut in the fourth from a clash of heads and being rocked late on, to take a 118-110, 116-112 twice triumph on the cards.
Adelaide Byrd’s score of 118-110 was massively harsh on Donaire who came on strong late on to push Magdaleno all the way. WBN scored the fight 115-114 to the champion who faces an uphill battle to regain his place at the top of the sport.
Earlier, Zou Shiming won a world title in just his tenth fight on the back of a glittering amateur career that included three amateur championships and two gold medals at the Olympics.
The likeable Chinese star totally dominated Prasitsak Phaprom at the Thomas & Mack Center in Las Vegas, dropping him in the second round of a whitewash through twelve one-sided rounds.
At the end of the contest, one judge somehow managed to score a round for Phaprom, although the others saw it similar to WBN. Final scores were 120-107 [2], 119-108 as Shiming scared a second unanimous win over Phaprom following an initial meeting two years ago.
Prior to the main event, power-puncher Oscar Valdez kept a firm grip on his WBO featherweight belt with a seventh round stoppage of an out-classed Hiroshige Osawa.