Photos showed Biden, 70, laughing with Carrie Johnson, 33, and little Wilfred near the ocean in Cornwall, where their husbands sat down ahead of the G7 summit.
The two women were spotted swapping their heels for flip-flops to frolic on the windy beach at Carbis Bay.
At one point, Biden and her British counterpart dipped their toes in the ocean, as Wilfred played on the sand in a diaper.
The first lady had on a black-and-white polka dot Brandon Maxwell dress and a black blazer with the word “Love” printed on the back, while Johnson, who married the prime minister last month, wore a long red frock.
The excursion took place after the two couples met for the first time earlier on Thursday, on the Bidens’ first full day of their European trip.
President Biden received a warm welcome in England from the British leader, but dodged his handshake to instead greet the PM’s new bride, joking that “we both married above our station.”
The heads of state swapped other compliments before moving to change the topic from a reported clash on Northern Ireland policy.
The meeting is the president’s first with a foreign leader abroad since taking office and will seek to underscore the “special relationship” between the UK and US.
During their summit, Biden and Johnson will announce a new Atlantic Charter, updating the 1941 agreement setting out US-UK goals for the world after World War II.