The move departs from Vietnam's tradition of shared leadership, in which the jobs have typically been held by different people, and echoes power structures in China under Xi Jinping and neighbouring Laos.
It has been widely expected since Lam's re-election as Communist Party head in January, when observers noted that his consolidation of party authority positioned him to assume the presidency as well.
Former central bank governor Le Minh Hung was elected as the country's prime minister for the next five years.
After being sworn in, the 69-year-old told the National Assembly that his top priority was to maintain peace and stability, which were the foundation for fast and sustainable growth.
"We aim to improve people's livelihoods so all can share the benefits of development," he said.
This is To Lam's second time holding both jobs, after briefly doing so in 2024 when his predecessor as party chief, Nguyen Phu Trong, died.
The concentration of power was significant since it meant that Lam had a "stronger mandate and far more political room to push through his agenda than any leaders" since the 1980s, when Hanoi launched reforms to shed a state-run economy in favour of a market-oriented one open to foreigners, said Nguyen Khac Giang, of Singapore's ISEAS–Yusof Ishak Institute research centre.
"The opportunity is obvious. Faster decision-making, greater policy coherence, and a better chance of pushing difficult reforms at a pivotal moment. But the risk is that concentration of power can move faster than institutional reform," he said.
Lam's rise to the top caps the ascent of a career policeman who advanced from Vietnam's security services to the top of the political system.
This was aided by a sweeping anti-corruption campaign launched by his predecessor, which he oversaw as head of the Ministry of Public Security.
As party chief, Lam has led Vietnam's biggest bureaucratic overhaul since the 1980s, cutting jobs, merging ministries, redrawing provincial boundaries and advancing major infrastructure projects.
He has focused on economic performance and private-sector growth, aiming to move Vietnam beyond the labour- and export-driven model that has helped lift millions from poverty and build a manufacturing-based middle class.
The country is targeting 10 per cent or higher annual economic growth over each of the next five years.
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