The American 10th seed played superbly to win 6-4 6-2 on Centre Court and set up a final - her first in the tournament's singles competition since 2009 - against Spain's Garbine Muguruza.
Konta, playing in only her second major semi-final, squandered two opportunities to break her opponent's serve in the ninth game.
The 37-year-old veteran then broke the home favourite in the final game to take the opening set 6-4 after 38 minutes.
Konta continued to struggle as Williams broke her opponent's resistance in the fourth game of the second set before a solid hold of serve enabled her to inch closer to victory.
The Briton then fell 0-40 down to offer up three set points in game 10, firing a backhand long on the second, and Williams got the decisive break at 3-1 in the second.
Konta could only watch as Williams became the oldest Wimbledon finalist since Martina Navratilova in 1994 by arrowing a forehand past her down the line on her third match point.