At a warm press conference when the two leaders glossed over differences on the Gaza Strip and wind power to present a united front, Trump said Russian President Vladimir Putin had "let him down" and he was disappointed other countries were still buying Russian oil because only a low oil price would punish the Kremlin.
After two days of his state visit to the United Kingdom, which the US leader described as an "exquisite honour," Trump was in a relaxed mode at the final press conference while Starmer was focused on avoiding areas of disagreement.
Neither leader was tripped up by potentially embarrassing subjects, with both batting away questions over the late sex offender Jeffrey Epstein and his ties to Trump and to Starmer's former ambassador to the US.
"We've renewed the special relationship for a new era," Starmer told reporters.
"This partnership today is a signal of our determination to win this race together and to ensure it brings real benefits in jobs, in growth, in lower bills, to put more hard-earned cash in people's pockets at the end of each month."
Trump also paid homage to the close ties enjoyed by the two countries, saying Starmer was a tough negotiator in securing the first tariff deal with the United States, although the UK has still not got the lower US tariffs on steel that it was seeking.
"We're forever joined and we are forever friends and we will always be friends," Trump said.
Earlier, at the start of a business reception, some of the leading names in US and UK business were welcomed by the two leaders to unveil a record 150 billion pound ($A309 billion) package of US investment into the UK, part of a wider 250 billion pound package officials say will benefit both sides.
Starmer said the deals would "light up the special relationship for years to come".
Trump was equally effusive.
"The ties between our countries are priceless," he said.
"We've done some things that financially are great for both countries ... I think it's an unbreakable bond we have, regardless of what we're doing today. I think it's unbreakable."
Starmer has pitched the UK as a destination for US investment, aligned to its financial services, tech and energy sectors so it can draw in US capital and build out its infrastructure to grow the economy.
He was keen to champion deals including a new technology pact with companies from Microsoft to Nvidia and OpenAI pledging 31 billion pounds and 100 billion pounds from Blackstone.
In terms of any differences, it was on foreign affairs where the two were most out of lockstep.
Starmer, and other European leaders, have been pressing Trump to put more pressure on Putin to end his war against Ukraine; Trump, while criticising the Russian president, failed to mention any further sanctions against Russia.
The US leader again expressed frustration that European countries were still buying Russian oil, but said Starmer was not part of their number.
Trump also repeated his position that he disagreed with countries recognising Palestine - something Starmer has said the UK will do if Israel fails to relieve suffering in the Gaza Strip and reach a ceasefire in its nearly two-year war with Hamas.
"I have a disagreement with the prime minister on that score, one of our few disagreements, actually," Trump said.
When the final question was answered, a clearly relieved Starmer ushered Trump out of the Great Hall at Chequers before the president leaves to return to Washington DC.
