The proposed budget eyes a $191 billion cut to the food stamp program which covers almost 42 million people.
Trump's proposal foresees selling half of the U.S. emergency oil stockpile, created in 1975 after the Arab oil embargo caused fears of price spikes.
All told, non-defense discretionary spending would be slashed by $1.6 trillion over the next decade, while defense discretionary accounts would be boosted by $489 billion.
The budget swiftly came under fire for proposing dramatic cuts to the social safety net.
"If we implemented this budget, you'd have to retreat from the world or put a lot of people at risk", said Republican Senator Lindsey Graham.
Republicans are under pressure to deliver on promised tax cuts, the cornerstone of the Trump administration's pro-business economic agenda, which would cut the business tax rate to 15 percent and reduce the number of personal tax brackets. Yet Mulvaney, a former SC congressman, apparently has little sway over his old Republican colleagues in the Senate, as many of them came out swinging against Trump's budget proposal only hours later.
Some top Republicans called the budget "dead on arrival", but House Speaker Paul Ryan said he's never seen a budget that didn't get the same reception.
"All POTUS budgets are", he tweeted.
"I mean I think this budget proposal as it's come forward would make every economic and social problem in Maine and America gravely worse", she says. Susan Collins, R-Maine, added.
That is likely to require a compromise between Republicans and Democrats over spending levels and priorities.
Another senior Republican lawmaker, Fred Upton of MI, questioned inclusion of money for Trump's border wall, remarking: "I thought Mexico was going to pay for the wall, why is this in our budget?"
The president's plan recommends $616bn in cuts to Medicaid, the government insurance programme for the poorest and many disabled Americans, while increasing border security spending by $2.6bn - including $1.6bn to begin construction on a wall along the US-Mexico border.
The budget would end subsidies for Amtrak to operate long-distance train service, cutting $630 million of the $1.4 billion in annual government support for passenger rail service.
Chris Hastedt, the public policy director for Maine Equal Justice Partners, says the cuts to social welfare programs are unprecedented and cruel.
Trump's budget summary includes a $610 billion Medicaid reduction over a decade, but as CNN pointed out, it's unclear how that ties into the budget's assumption that the American Health Care Act becomes law, carrying with it an $800 billion cut to Medicaid, a program that provides health coverage for low-income Americans. "It's not right when you look at it through the perspective of the people who pay taxes", he said.
But anti-poverty groups note that while many food stamp recipients already work, they are unable to earn enough money to live on. "And we will do that".
Lawmakers from both sides of the aisle said the Defense Department baseline budget proposal of $603 billion for fiscal 2018 unveiled on Tuesday by the White House Office of Management and Budget didn't offer significantly more than Obama had initially proposed for the same period and would face a major overhaul in the House and Senate. "We're going to measure compassion by the amount of money that we spend, but by the number of people that we help".
Trump has long said he was suspicious of climate change.
The budget chief argued that President Barack Obama's administration spent too much on climate change programs and that the 2018 proposed cuts are an attempt to get back to normalcy.