。
Mary was the only one who came.
Literal
Come-[attributive-た] [nominalizer-の] [topic-は] Mary only was.
のは is the cleft structure — here marking 'the [one] who came' as topic, with the comment 'was only Mary.' This English-style emphasis on a specific subject is elegantly done through cleft in Japanese. だけ ('only') restricts to Mary alone.