Poland's world No. 2 Iga Świątek lost 4-6, 4-6 to Coco Gauff in the women's singles, while Hubert Hurkacz was edged out by Team USA's Taylor Fritz 4-6, 7-5, 6-7 in a closely contested men's singles match.
With these two wins, the Americans secured an unassailable lead, and the mixed doubles match was not played, Polish state news agency PAP reported.
Team Poland also included Jan Zieliński, Maja Chwalińska, Alicja Rosolska and Kamil Majchrzak.
En route to the championship clash, Poland defeated Noway, the Czech Republic, Britain and Kazakhstan.
The encounter against Team USA marked Poland's second straight United Cup final after its runner-up finish to Germany last year.
The United Cup serves as a warm-up event for the Australian Open Grand Slam tournament, which is set to begin in Melbourne on January 12.
(gs)
Source: IAR, PAP, Polsat Sport