Jane Mendillo is returning to Harvard Management Co. on July 1 as president and CEO after a six-year hiatus. Her role oversees the investments of Harvard University’s $34.9 billion endowment. Mendillo remains the CIO of the $1.7 billion Wellesley College endowment through June, while the college’s president and trustees work on a search plan. In her new role at Harvard Management, she replaces Mohamed El-Erian, who left late last year to return to PIMCO as co-CEO and co-CIO. Previously Mendillo worked with Harvard Management for 15 years, where she was vice president of external management before leaving in 2002 to join Wellesley.