Satya Nadella is Microsoft Corp.'s new CEO. But he ascends as co-founder Bill Gates returns to a more central role at a company struggling to catch up in mobile and other fast-moving technologies.

Mr. Gates is giving up the chairman's role to become technical adviser to Mr. Nadella, the 22-year Microsoft veteran who was named Tuesday to succeed Steve Ballmer as its third chief executive. Mr. Gates said he would devote up to a third of his time—which is now largely devoted to philanthropic activities—to help Mr. Nadella craft...