During the 166.5 kilometre stage between Nice and Valdeblore La Colmiane, Benoot gradually hacked away at the advantage.
However Schachmann responded during a 16km climb to the finish line where he collapsed to the ground in exhaustion.
"The last three kilometres were hell," said the 26-year-old German. "But now I'm in heaven. This is the greatest victory of my life."
Schachmann of the Bora-Hansgrohe team completed the Paris-Nice course in 27 hours,14 minutes and 23 seconds.
Epic
The 78th edition of the race has been one of the few sporting events to escape cancellation or postponement due to the spread of the coronavirus throughout the world.
Question marks hang over the Paris-Roubaix event. The 257 km race is scheduled for 12 April. There are also fears for the one-week Criterium de Dauphine, which snakes through the Alps in early June.
Schachmann, who won the opening stage outside Paris on 8 March, tightened his grip in the time-trial three days later. He is the first German since Tony Martin in 2011 to win the race and also the first since compatriot Jorg Jackshe in 2004 to have retained the leader’s jersey from start to finish.
Colombia's Sergio Higuita finished third overall while the pre-race favoruite Nairo Quintana finished seventh.
Daily news briefReceive essential international news every morning
Subscribe
