 By the time of the death of Walter Comyn, Lord of Badenoch, jure uxoris Earl of Menteith in 1258, she may have already been married.In 1260, Isabella was arrested, along with her new husband, an English knight called John Russell, for the poisoning of her late husband; by the month of April 1261, Walter and Mary were ruling the province as Count and Countess.She died before her husband, but the exact year is not known.