WATERLOO, Ontario—After 20 years together at the helm of Research In Motion Ltd., Jim Balsillie and Mike Lazaridis, the company's co-chief executives, turned over the top job late Sunday to a little-known insider as part of a board and management shuffle.

For months, investors have clamored for a significant strategic overhaul, fresh leadership or a sale of the company as the BlackBerry maker struggled to stay competitive with rivals Apple Inc. and Google Inc. amid operational blunders and a tumbling share price. The surprise...