Namesakes
Belgian Prime Ministers:
1 prime minister
Paul van Zeeland (a.k.a. Guillaume) 1935-1937 
French Presidents and Prime Ministers:
2 prime ministers
(prime minister) François Guizot (a.k.a. Guillaume) 1847-1848
(prime minister) Charles Cousin-Montauban, Comte de Palikao (a.k.a. Guillaume) 1870
Haitian Presidents:
2 presidents
(president) Fabre Geffrard (a.k.a. Guillaume) 1859-1867
(president) Vilbrun Guillaume Sam 1915
Notable Actors and Entertainers:
2 actors
Guillaume Depardieu 1971-2008
Guillaume Canet 1973-
Notable Athletes:
2 handball
(handball) Guillaume Gille 1976-
(handball) Guillaume Joli 1985-
Notable Musicians:
1 composer
Guillaume de Machaut c. 1300-1377
Notable Politicians and Statespeople:
1 politician
Jan Marijnissen (a.k.a. Guillaume) 1952-
Notable Writers:
2 poets
Guillaume de Machaut c. 1300-1377
Guillaume Apollinaire 1880-1918
Olympic Medalists:
2 gold, 1 bronze, 1 silver
(silver) Guillaume LeBlanc 1992 20 km walk
(gold) Guillaume Gille 2008; 2012 handball
(gold) Guillaume Joli 2012 handball
(bronze) Jean-Guillaume Béatrix 2014 biathlon
Other Royalty:
2 grand dukes, 1 prince
Jean, Grand Duke of Luxembourg (a.k.a. Guillaume) 1964-2000 Luxembourg
Henri, Grand Duke of Luxembourg (a.k.a. Guillaume) 2000- Luxembourg
Emmanuel of Belgium (a.k.a. Guillaume) 2005- Belgium
Saints:
7 blessed, 5 saints
Saint William Firmatus (a.k.a. Guillaume) 1026-1103
Saint William Breteuil (a.k.a. Guillaume) ?-1130
Saint William of Bourges (a.k.a. Guillaume) 1149-1209
Blessed William of Maleval (a.k.a. Guillaume) ?-1157
Saint William Pinchon (a.k.a. Guillaume) 1175-1234
Saint William of Pontoise (a.k.a. Guillaume) ?-1192
Blessed William Tempier (a.k.a. Guillaume) ?-1197
Blessed William Arnaud (a.k.a. Guillaume) ?-1242
Blessed Guillaume of Toulouse 1297-1369
Blessed Guillaume Saultemouche 1555-1593
Blessed Guillaume Répin 1709-1794
Blessed Guillaume-Joseph Chaminade 1761-1850