Lawler's departure piles pressure on BP's Chairman Helge Lund and board as they seek to project stability in the wake of Looney's abrupt resignation earlier this month after less than four years in the top job for failing to fully disclose details of past personal relationships with colleagues.
The United States is the largest single country for BP's operations. It includes oil and gas production in the Gulf of Mexico and onshore shale basins, several refineries as well as large investments in offshore wind, biogas and retail.
Lawler, 55, joined BP in 2014 and has led BP's shale business, known as BPX Energy, since.
He oversaw BP's $10.5 billion acquisition of BHP's onshore shale assets, which turned it into one of the top producers in the Permian oil basin.
Lawler, chairman and president of BP America, is leaving "to pursue new career opportunities," the memo seen by Reuters said. The FT was first to report the news.
Orlando Alvarez will replace Lawler as head of BP America. Alvarez will continue in his role as senior vice-president gas and power trading, Americas. Kyle Koontz will take over as chief executive officer for BPX energy, the memo said.
