King posted one minute 4.13 seconds to hold off Efimova in a repeat of their acrimonious Olympic final and eclipse the world mark set by Lithuania's Ruta Meilutyte in 2013.
"That race was always going to be a showdown, an exciting dog fight," King told reporters.
"We get a lot of rivalries like this in football, basketball, swimming where we see a lot of really nice people, being really nice," she added.
"That’s great and all but it’s not my personality. I’m spunky, I’m confident and I’m not going to not be myself before a race."
The U.S. secured a memorable one-two as Katie Meili touched for silver in one minute 05.03 seconds.
King, 20, has voiced her disapproval that Efimova is allowed to compete at global level after the Russian was twice caught using banned substances.
The spiky relationship continued in Budapest as King celebrated her victory wildly and did not exchange eye contact with Efimova who took bronze, 0.02 seconds behind Meili.
King's strong first 50m set the tone and she easily held on to beat Meili by 0.90 seconds before the pair embraced warmly and Efimova was left to smile by herself.
(Editing by Ed Osmond)
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