He is named in the Cartulaire de Saint-Bertin, which also specifies his parentage. After the election of Guillaume de Normandie as count of Flanders in Mar 1127, nobles in Gent invited Thierry d'Alsace to intervene. Orderic Vitalis records that he captured Lille, Furnes and Gent. He was recognised as count at Bruges 30 Mar 1128.
After Count Guillaume died from injuries received at the battle of Aalst 28 Jun 1128, Thierry was generally accepted as THIERRY I Count of Flanders. He went to Palestine in 1138, and joined Louis VII King of France in Jun 1147 on the Second Crusade. The Annales Blandinienses record that he returned to Palestine in 1157 and in 1164, taking part in campaigns with Baudouin III King of Jerusalem during the earlier visit.
The Flandria Generosa specifies that "Theodericus comes monarchiam Flandrie" was buried in "cenobio Watinensi". The necrology of the Prieuré de Fontaines records the death "18 Jan" of "Teodericus comes Flandrie, pater domine Matildis". 1
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