If you look carefully at Act III scene 1, we can see that Mark Antony is carefully removed from the scene by Trebonius before the conspirators start their action against Caesar, culminating in his assassination. Examining the text, we see that Cassius fears the discovery of the plot, but then he is reassured by the way that Trebonius does what he had to do by leaving with Mark Antony:

Trebonius knows his time; for look you, Brutus,
He draws Mark Antony out of the way.

Thus it is that Mark Antony, as planned by the conspirators, is absent from the scene when the murder is committed, and is therefore not present to witness the assassination of his leader. This of course, as we go on to see, is a decision that backfires disastrously on the conspirators. The way that Brutus underestimates Mark Antony and ignores the advice of Cassius to kill him too results in his own downfall.