Down to their last strike when Jarrod Dyson kept the inning alive by lacing a double to right, the Royals knotted the game at 6 on Donald's two-run shot to right off reliever Donn Roach.
Will Venable had knocked in the go-ahead run for the Padres as part of a three-run seventh inning.
Yonder Alonso started the Padres' scoring with a solo home run in the fifth inning, his second of the spring.
The Padres, who trailed 4-0 after four innings, got three runs back in that fifth, as Kyle Blanks smacked a two-run home run. It was Blanks' first home run this spring.
The Padres tied the score at 4 when Everth Cabrera walked with the bases loaded in the seventh inning.
Carlos Peguero belted his first Cactus League home run and Justin Maxwell continued his hot spring for the Royals with three hits, lifting his average to .419. Salvador Perez, Norichika Aoki and Omar Infante all had two hits in the loss.
San Diego starter Ian Kennedy allowed a run in the first inning of his third consecutive start as Infante drilled an RBI double to the left-center-field gap to score Aoki, who singled to start the game.
Kennedy allowed runs in the third, fourth and fifth innings. He allowed four runs on eight hits with two walks and one strikeout. He threw 80 pitches -- 51 for strikes. He's allowed 20 hits in 13 2/3 innings this spring.
Kansas City starter Jason Vargas was cruising along, having allowed one hit over four scoreless innings before the Padres got to him for the three runs in the fifth inning. He allowed four hits with three runs, one walk and had four strikeouts.
San Diego reliever Joaquin Benoit tossed a scoreless inning and hasn't allowed a run in six innings over six appearances.
Up next: Yordano Ventura, the 22-year-old hard-throwing right-hander, will try to enhance his chances of breaking into the Royals' rotation when he starts against the Rangers at 8:05 p.m. CT on Monday at Surprise Stadium. The game, which will be broadcast live on MLB.TV and on FOX Sports Kansas City, will be the fourth Cactus League contest in which the expanded instant replay procedure will be available to the Royals. Ventura has been pitted against left-hander Danny Duffy for the last spot as a starter and has a 2.89 ERA in his first three outings. Also possibilities to pitch are Kelvin Herrera, Louis Coleman, Greg Holland, Donnie Joseph and Cory Wade. The Rangers will start Matt Harrison. Look for Alex Gordon, Eric Hosmer and Mike Moustakas, all of whom had the day off on Sunday, to be in the Royals' lineup. The two teams, which share the Surprise training complex, split their first two games this year.