He entered the Benedictine order of the Congregation of Cluny in the monastery of Saint-Martin des Champes in Paris; for a some time, he was a monk at Cluny. Later, he became abbot of the monastery S. Maria Nuova in the diocese of Poitiers. He was a friend of St. Bernard of Clairvaux, who mentioned him in his letters. He was created Cardinal-Bishop of Tusculum (Frascati) by Pope Innocent II in the consistory celebrated in 1142 and received the episcopal consecration from that pope in the Lateran Basilica in March of that year. He participated in the papal elections of September 1143, March 1144, July 1153 and December 1154, and subscribed the papal bulls issued between April 19, 1142 and February 18, 1159. He served as papal legate in England in the pontificate of Pope Lucius II (1144–1145). He became Dean of the Sacred College of Cardinals in 1153. After the double papal election, 1159 he joined the obedience of Antipope Victor IV and consecrated him to the episcopate in the abbey of Farfa on October 4, 1159 with the assistance of the bishops Ubaldo of Ferentino and Riccardo of Melfi. He was deposed by the legitimate Pope Alexander III. He participated in the schismatic Council of Pavia in February 1160, which anathematized Alexander III. Then he retired to the abbey of Cluny and died there.
Spanish historian Alphonso Ciacconio (c.1540–1599) in his opuscle Vitae et res gestae Pontificum Romanorum et S. R. E. Cardinalium mentions him among participants of the election of Antipope Paschal III (successor of Victor IV) in April 1164, but this is unlikely because up to that time he had been replaced in his suburbicarian see..
- Johannes M. Brixius, Die Mitglieder des Kardinalskollegiums von 1130-1181, Berlin 1912, p. 44 and 91-92
- Gaetano Moroni, Dizionario di erudizione storico-ecclesiastica da S. Pietro sino ai nostri giorni. Vol. XXXIII, Tipografia Emiliana, Venezia, 1840 - 1861, p. 266-267
- Philipp Jaffé, Regesta pontificum Romanorum ab condita Ecclesia ad annum post Christum natum MCXCVIII, Berlin 1851
- Biographical entries of the Cardinal-Bishops of Frascati (Tusculum) on the official site of that suburbicarian see
- Salvador Miranda: Consistories for the creation of cardinals XII Century (1099-1198) and Papal elections of the XII Century (1100-1198)
- Brixius, p. 44, says that he died between April 7, 1161, when he is attested for the last time as a living person, and June 1, 1162, when Theobaldus was appointed his successor as cardinal-bishop of Frascati. Date October 28 without a year is given by the necrology of St. Martin des Champs, published by F. Duchesne, Histoire de tous les cardinaux françois, 1660, p. 99
- This is according to the site of the see of Frascati and Brixius, p. 44. G. Moroni, p. 266 indicates that he was not abbot of S. Maria Nuova but abbot general of the Congregation of Cluny; however, Moroni seems to be in error because Icmar is not mentioned in Catalogus abbatum Cluniacensium
- G. Moroni, p. 266-267; Site of the see of Frascati
- S. Miranda: List of the cardinals created by Innocent II
- G. Moroni, p. 266
- S. Miranda: papal elections of 12th century
- Philipp Jaffé, p. 559, 605, 609, 615, 653 and 658
- G. Moroni, p. 266
- S. Miranda, Election of Antipope Victor IV, September 7, 1159
- S. Miranda: pseudocardinal bishops of Victor IV (no. 2 and 3)
- S. Miranda: Notes to the election of Antipope Paschal III on April 22, 1164
- Brixius, p. 44. Neither G. Moroni, p. 266-267, nor the site of the see of Frascati mention Icmar's participation in the election of this antipope.