In 924 or 925, he became the second husband of Marozia, a Roman noblewoman who had the title senatrix patricia Romanorum.
In order to counter the influence of Pope John X (whom the hostile chronicler Liutprand of Cremona alleges was one of Marozia's lovers), Marozia subsequently married his opponent Guy of Tuscany, who loved his beautiful wife as much as he loved power.
Together they attacked Rome, arrested Pope John X in the Lateran, and jailed him in the Castel Sant'Angelo.
Either Guy had him smothered with a pillow in 928 or he simply died, perhaps from neglect or ill treatment.
None of his children survived him and when he died in 928 or 929 his brother Lambert succeeded him as count and duke of Lucca and margrave of Tuscany.