Lloyd Blankfein is preparing to step down as Goldman Sachs Group Inc.’s chief executive as soon as the end of the year, capping a more than 12-year run that has made him one of the longest-serving bosses on Wall Street.

Goldman is likely to follow an announcement of Mr. Blankfein’s departure with a quick transfer of power and isn’t looking beyond Goldman’s two co-presidents, Harvey Schwartz and David Solomon, to replace him, people familiar with the matter said.

The timing of any moves could still change, and the 63-year-old Mr. Blankfein is firmly in control of his exit, the people said. The current thinking, though, is that he will retire ahead of or early in Goldman’s 150th anniversary year in 2019, a fitting send-off for the history buff.

Mr. Blankfein has often joked he will die at his desk, and his enthusiasm for the job has led many within the firm to believe he might outlast another set of would-be successors. Gary Cohn joined the Trump administration after tiring of life as Mr. Blankfein’s understudy. That set off the promotions of Messrs. Solomon and Schwartz and a new chapter in Goldman’s succession planning.

That planning effort has intensified of late, people familiar with the matter said, as expectations have risen among top executives and board members that the clock has started ticking on Mr. Blankfein’s tenure.