Agnelli was a pupil of the well-known sculptor Nicola Pisano, who modelled on classical Greek and Roman ideas.
He was soon engaged in work on the convent of the brethren at Pisa and built the campanile of the Badia a Settimo, near Florence.
His best work is the series of marble reliefs executed, in collaboration with Pisano, for the famous tomb of St Dominic in the church of that Saint at Bologna.
Agnelli's work on the posterior face of the tomb deals with six Dominican legends: the Blessed Reginald smitten by a distemper; the Virgin Mary healing a sick man and selecting the habit for the Friars Preachers; the same man freed from a terrible temptation by holding St Dominic's hands; Pope Honorius III having his vision of St Dominic supporting the falling Lateran Basilica; Honorius examining the Dominican rule, and his solemn approbation of it.
In 1304, he was engaged in works of sculpture and architecture in his native Pisa, and was called upon to adorn the facade of the Church of San Michele di Borgo with historical bas-reliefs.