Overall it was a pretty routine performance from Watford as Palace offered little to impress new boss Neil Warnock.
Indeed, Richard Lee in the Watford goal had a mundane evening without having to make one serious save in the entire 90 minutes.
Watford eased their way to a comfortable three points leaving Palace loitering in the bottom three.
Palace made a promising start though with Portsmouth loanee Franck Songo'o giving them plenty of options down the right flank.
However, Watford gradually took control as Darius Henderson headed wide and Danny Shittu shot just wide before Smith opened the scoring on 32 minutes.
From a Lloyd Doyley throw-in, King's shot was blocked but it rebounded kindly to Smith who made no mistake with a low shot from a fine angle which beat Julian Speroni via the near post.
There was to be no trouble for Watford after the break despite plenty of spirit from the Palace side.
On 67 minutes Adam Johnson made a fine run down the left before dragging the ball into the path of King located on the edge of the penalty box and the hitman drilled a low shot past Speroni and into the corner of the net.