Younson, head of employment law at Baker & McKenzie, will join McDermotts in April to head the office's employment practice.
His appointment is one of three announced this week as part of the firm's plan to build a full-service London practice.
Philip Rees is joining McDermotts' intellectual property practice from Citibank, where he was senior technology counsel, Europe.
McDermotts has also confirmed that John Reynolds, a partner in Herbert Smith's litigation group, has become the first partner in its commercial litigation practice, as reported in The Lawyer last week.
The appointments, according to partner in charge William Charnley, are part of the firm's plans to increase the number of lawyers in its London office "five-fold over the next 90 days".
"Younson is known as one of the two doyens of employment law, and these are the sort of people we will be talking to over the next few weeks," says Charnley.
Meanwhile, US firm O'Melveny & Myers has appointed Adrian Harris, former deputy head of Richards Butler's corporate and commercial group, as a partner in its London finance practice.