The diocese was established in 589 AD: the fictitious chronicles pretended that St. Sacerdos of Limoges in France had been its bishop; Protogenes was present as Bishop of Sigüenza at the Third Council of Toledo and again the same Protogenes at Gundemar's council in 610; Ilsidclus assisted at the fourth, fifth and sixth councils; Wideric, at the seventh to the tenth; Egica, at the eleventh; Ela, at the twelfth, thirteenth and fourteenth; Gunderic, at the fifteenth and sixteenth. The succession of bishops continued under the Arab domination: Eulogius of Córdoba, in 851, was succeeded by Sisemund. But later on Sigüenza was so completely depopulated that it does not appear among the cities conquered by Alfonso VI of Castile (1065-1109) when he subdued all this region. The first bishop of Sigüenza, after it had been repeopled, was Bernardo, a native of Agen in France, who had been "capisol" (caput schola, Latin for school head(master)) of Toledo; he rebuilt the church and consecrated it on the Feast of St. Stephen, 1123, and placed in it a chapter of canons regular; he died Bishop-elect of Santiago. On 14 March 1140, Alfonso VII granted the bishop the feudal lordship of Sigüenza, which his successors retained until the fourteenth century, making the diocese a minor prince-bishopric.
After the long episcopate of Bernardo, Pedro succeeded, and was succeeded by Cerebruno, who began the building of the new cathedral. Jocelin, an Englishman, was present with the king at the conquest of Cuenca; he was succeeded by Arderico, who was transferred to Palencia; Martín de Hinojosa, the Abbot of Huerta, abdicated the see in 1192, and was succeeded by Rodrigo.
In 1465 Diego López of Madrid, having usurped the mitre, fortified himself there. Pedro Gonzalez de Mendoza, the Crown Cardinal of Spain, held this diocese together with the archbishopric of Toledo, and enriched his relations by providing establishments for them at Sigüenza. His successor, Cardinal Bernardino de Carvajal, was dispossessed as a schismatic by pope Julius II for his share in the Conciliabulum of Pisa. Bishop of Sigüenza Fadrique de Portugal y Noroña became in the first third of the 16th Century, Archbishop of Zaragoza and Viceroy of Catalonia. Bishops Garcia de Loaisa, Fernando de Valdés y Salas, Pedro Paeheco and others held this wealthy see. The castle-palace, modified in various ways, suffered much from the storms of civil war, and was restored by Joaquin Fernandez Cortina, who was bishop from 1848, and the restoration was continued by Bishop Gomez Salazar (1876–79).
In 1959 it was established as Diocese of Sigüenza-Guadalajara.
Ordinaries
Rodrigo (1192–1221 Died)
Juan (3 Jun 1300 – )
Arnaldo (17 Nov 1326 – 1328 Died)
Blasco Dávila (1340–1341 Died)
Lope Rodrigo de Villalobos (29 Oct 1382 – 21 Jun 1386 Died)
Juan de Serrano (22 Dec 1389 – 24 Feb 1402 Died)
Juan Gonzalez Fernandez de Illescas (30 Jul 1403 – 15 Nov 1415 Died)
