Horse locomotion is remarkably economical. Here we measure external mechanical work of the galloping horse and relate it to published measurements of metabolic cost. Seven Thoroughbred horses were galloped (ridden) over force plates, under a racing surface. Twenty-six full strides of force data were recorded and used to calculate external mechanical work of galloping. The mean sum of decrements of mechanical energy was -876J (± 280J) per stride and increments were 2163J (± 358J) per stride as horses were accelerating. Combination with published values for internal work and metabolic costs for galloping yield an apparent muscular efficiency of 37-46% for galloping which would be reduced by energy storage in leg tendons. Knowledge about external work of galloping provides further insight into the mechanics of galloping from both an evolutionary and performance standpoint.