Giovanni Boccamazza began his career in the Church as a Canon of the Vatican Basilica[6] and Chaplain of Pope Nicholas III (1277-1280).
The Chapter of the Cathedral of Monreale had originally and unanimously chosen Guillaume, Bishop of Potenza, but he declined the position and resigned the election into the hands of the Pope.
Nicholas then received the bishop's resignation of the election to Monreale and sought to appoint Bernard de Montemirato, OSB, the Abbot of Montismajoris in the Diocese of Arles.
In reply Honorius sent Cardinal Giovanni Boccamazza as Apostolic Legate to Germany, Bohemia, Hungary and Sweden to engage in conversations with Rudolf.
[17] The case was finally disposed of on 3 October 1291 with the suspension of the Bishop from his pontifical powers and right to collate to benefices for three years.
[18] Also, during the same Legatine assignment, Cardinal Giovanni became involved in strife between the Dominicans and the people of Strasbourg, and he felt compelled to lay the city under the Interdict.
[30] On 5 March 1291 Pope Nicholas granted Cardinal Giovanni Boccamazza the power to investigate and reform the apparently large number of encroachments and illegal occupations made by various persons upon the properties belonging to the Lateran Basilica.
[31] The same service was imposed on Cardinal Giovanni with respect to the Premonstratensian monastery of S. Quirico de valle introducti in the diocese of Reate.
[34] On 12 March 1296 he was one of twenty-one cardinals who received a 1/18 share in the payment of 2000 livres Tournois presented by the Procurators of the Abbot of Cluny.
[42] On 5 September 1298 there was a distribution of the payment of 180 livres Tournois made by the Abbot of the Major Monasterium in Tours, with a 1/15 share coming to Cardinal Giovanni of Tusculum.
On 1 February 1301 Cardinal Giovanni of Tusculum, who had been acting as Assessor (judge) on appointment by Boniface VIII, pronounced his verdict in the case of the contested election of Leo, Bishop of Soli in Cyprus.
[46] On 20 September 1301 his name heads the list of fifteen cardinals, ahead of Bishops Theoderic (Civitas Papalis) and Leonardo (Albano), subscribing to the papal grant of privileges to the Monastery of Santa Croce Saxivivo in the Diocese of Foligno.
[48] On 2 July 1302 he is noted as having on a committee which examined the election and person of John, Abbot of Holy Cross Monastery in Waltham in the Diocese of London.
[51] He participated in five papal conclaves, which saw the elections of popes Nicholas IV in 1288, Celestine V in 1294, Boniface VIII in 1294, Benedict XI in 1303 and Clement V in 1305.
[52] In 1304 Cardinal Giovanni was again a member of a committee of cardinals that examined the election of an abbot for the Benedictine monastery of S. Maria de Alfiolo in the Diocese of Gubbio;[53] and likewise for the Benedictine Monastery of S. Angelo de Gaifa in the Diocese of Urbino;[54] On 4 March 1304 Pope Benedict XI (1303-1304) granted Cardinal Giovanni Boccamazza subinfeudation of three castles, Scandrillia, Castellucii, and Rocca Soldana, which belonged to the Monastery of Farfa by primary infeudation of the Roman Church, along with their tenements and vassals, with full jurisdiction and authority.