Janssen won the Tour de France in 1968 in front of Herman Van Springel and Ferdinand Bracke of Belgium.

Janssen was also 2nd to Lucien Aimar of France in the 1966 Tour de France.

Janssen won the Tour de France Points Jersey three times, in 1964, 1965, and 1967.

Janssen’s sole victory in the Tour was as follows. Going into the final stage of the 1968 Tour de France, Janssen was behind the race leader, Herman Van Springel, by 16 seconds. The last stage was a 34 mile (55 km) individual time trial.

Janssen rode the race of his life. He not only won the time trial, but won the overall race by 38 seconds over a demoralized Van Springel.

That year, Janssen became the first Dutchman to win the Tour. He also won the Tour while not wearing the leader's jersey anytime during the race except after the final stage.

In other Grand Tour events, Janssen won the Vuelta a Espana in 1967 over Jean-Pierre Ducasse of France and Aurelio Gonzales of Spain.

Janssen also won the Points Jersey twice in the Vuelta a Espana in 1967 and 1968.

In one-day events, Janssen won the World Championship Road Race in 1964 over Vittorio Adorni of Italy and Raymond Poulidor of France. Janssen also was 2nd in 1967 to Eddy Merckx of Belgium.

Janssen won the Paris-Roubaix Classic in 1967 in front of Rik Van Looy of Belgium and Rudi Altig of Germany.

Janssen was also 2nd in the 1966 Paris-Roubaix behind Felice Gimondi of Italy, and was 3rd in the 1963 race behind Emile Daems and Rik Van Looy of Belgium.

In addition, Janssen won the Championship of Zurich in 1962.