The NewsGuild of New York will represent about 150 employees of NBC News Digital.
Greg Burton accused union sympathizers of "surveilling" other journalists, saying that's something he'd expect of a murderer or child molester.
"It's about damn time," a letter from the organizing committee said.
Employees of the subscription news outlet voted to authorize a strike after more than two years of bargaining.
"We believe having a union will help us protect and elevate the good conditions we currently enjoy in our workplace, both for ourselves and for all who come after us," staffers said.