In September 2015, Al-Rumayyan was appointed managing director of the Public Investment Fund of Saudi Arabia, with a mandate to seek international investments with the potential for knowledge and technology transfer to the Saudi economy.
In June 2016, PIF invested $3.5 billion in the US-based Uber Technologies Inc. In a statement on the deal, Al-Rumayyan cited the importance of increased urban mobility and local employment in support of the Saudi Vision 2030 economic reform program. He joined Uber's board of directors as part of the agreement.
In August 2016, Al-Rumayyan joined the board of the state oil company Saudi Aramco as part of a move toward closer cooperation between Aramco and PIF in restructuring the Saudi economy. In October that year, PIF under Al-Rumayyan invested $45 billion in the newly established SoftBank Vision Fund, a technology-focused investment vehicle. A further overseas investment by PIF followed in May 2017 with a $20 billion contribution to a joint US infrastructure fund with Blackstone.
In October 2017, Al-Rumayyan announced that PIF would aim to increase its assets under management to more than $400 billion and create more than 20,000 new jobs by 2020. At the same time he presented a new investment strategy for the fund, based on four objectives of maximizing assets, investing in new sectors, localizing technologies, and developing economic partnerships. He also said PIF would borrow conservatively to finance specific assets and would seek further partnerships such as those with Blackstone and SoftBank.
In September 2018, Al-Rumayyan announced PIF's first step in incorporating loans and debt instruments into its long term strategy, with an $11 billion loan facility. The following month he said the fund would aim to increase the percentage of international assets in its portfolio from 10% to 50% by 2030.
At the Milken Institute in Abu Dhabi in February 2019, Al-Rumayyan announced that PIF would open new offices in London, New York and San Francisco, and would increase its workforce from 450 to 700 by the end of 2019. He also said the fund would invest in renewable energy projects in Saudi Arabia, including the local manufacture of solar panels.