"It was her/him who did so-and-so" often sounds a bit more natural in ordinary colloquial speech. And as a short answer to the question "Who did it?" or "Who arrived first?", it's MUCH more natural to say "It was him [точка]" or "It was her [восклицательный знак]" That is, without a relative clause introduced by "who".

If you say "Who did it?" -- "It was he!", it sounds rather pompous, and you may remind people of Bela Lugosi in the 1931 version of Dracula. (He's always parodied as speaking with hyper-correct grammar: "It is I, Count Dracula!"; though 99.99% of English speakers would say, for example, "It's me, Buffy Summers.")

However, as zedeeyen wrote, "It was he/she who..." is the form preferred by traditional grammar, and thus it should be used in formal writing (or in highly formal speech, such as an academic lecture).