“The pen is mightier than the sword” was coined by Edward Bulwer-Lytton in for his play Richelieu; Or the Conspiracy, 1839:

True, This! –
Beneath the rule of men entirely great,
The pen is mightier than the sword. Behold
The arch-enchanters wand! – itself a nothing! –
But taking sorcery from the master-hand
To paralyse the Caesars, and to strike
The loud earth breathless! – Take away the sword –
States can be saved without it!

Bulwer-Lytton may have coined the phrase but he was preceded by several others who expressed essentially the same idea:

George Whetstone, in Heptameron of Civil Discourses, 1582, wrote “The dashe of a Pen, is more greevous than the counterbuse of a Launce.”

In Hamlet, 1602, Shakespeare gave Rosencrantz the line “… many wearing rapiers are afraid of goose-quills and dare scarce come thither.”

Robert Burton’s The Anatomy of Melancholy, 1621 includes “From this it is clear how much more cruel the pen may be than the sword.”

Thomas Jefferson sent a letter to Thomas Paine in 1796, in which he wrote: “Go on doing with your pen what in other times was done with the sword.”

HT: http://www.phrases.org.uk/meanings/the-pen-is-mightier-than-the-sword.html