[2] He won the singles title at the Dutch National Tennis Championships in 1905, 1907, 1919, 1921, 1925 and 1926.

[3] Van Lennep participated in the singles and doubles events at the 1908 and 1924 Olympics.

[4][5] He reached the final of the singles event at the inaugural British Hard Court Championships, played in Torquay in April 1924, in which Randolph Lycett defeated him in four sets.

[6] He was more successful in international doubles competitions as he reached the semifinal of the 1926 Wimbledon Championships partnering Béla von Kehrling.

[3] Van Lennep played in 12 ties for the Dutch Davis Cup team between 1920 and 1928 and has a won-loss record of 14 to 12.