As a young man, staying in the house of Ugolino de' Conti while observing Lent in Rome in 1215 or 1216, he encountered Dominic of Osma.
They discussed Christian ministry, and talked of future missionary work among Prussians or other northern pagans.
Leaving Paris in July, Dominic and William travelled with a third Friar, known only as John (Iohannes), to Milan and Bologna.
William now remained with the Pope and gained his close trust, but was at Bologna in summer 1221 when Dominic died.
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