Sir Keir has been at pains to charm Mr Trump into striking a good trade deal with the UK, following the wilderness years under former president Barack Obama when the UK was famously "at the back of the queue".
Sir Keir's defining mission on this trip has been to try and persuade Mr Trump to provide a security guarantee for Ukraine in any peace deal reached with Russia, so that Vladimir Putin does not invade its neighbour again following an initial ceasefire.
Asked if Sir Keir had convinced him not to impose trade tariffs on the UK, Mr Trump said to laughter: "He tried.".
More praise came when Sir Keir said the UK was "ready to put boots on the ground and planes in the air" to support a deal on Ukraine working with allies.
As part of that, the prime minister is hoping the US will not impose punitive tariffs on the UK, as it minded to do with close neighbour Canada - because any such move would make his mission to grow the economy nigh on impossible.
