Pitney, 65, apparently died in his room in Cardiff of natural causes, police said.
"Last night was one of the best performances, not vocally, but from the enthusiasm. He just wanted to please - and he did," said Wendy Horton, who reviewed last night's concert in Cardiff for the South Wales Echo newspaper.
"It came through in his voice because he really let it rip. If you are ill, that would be one of the first things to show it," said Nigel Corten, who reviewed the show for the South Wales Argus. "The audience were in raptures.”
During a long career, Pitney had hits as a singer - Town Without Pity, (The Man Who Shot) Liberty Valance, Half Heaven, Half Heartache - and as a writer, penning Hello Mary Lou for Ricky Nelson and Rubber Ball for Bobby Vee.
In 1962 Pitney had the top two songs on the US chart - his rendition of Only Love Can Break a Heart was at number two, just behind a song he wrote, He's a Rebel for The Crystals.
He was inducted into the Rock and Roll Hall of Fame in 2002.
Pitney had some success as a country singer, pairing with
George Jones to record I've Got Five Dollars and It's Saturday Night and Louisiana Man.
He also took second place twice at the San Remo Song Festival in Italy, and had a regional hit with Nessuno Mi Puo' Giudicare.
"I'm a performer," Pitney said in an interview with The Associated Press in 1997. "I've tried everything there is. I don't know how you can label something like that."
Pitney married his high school sweetheart, Lynne, in 1967, and kept a base in Connecticut all his life. He built a recording studio in his home at Somers, 30 kilometres northeast of Hartford.
Pitney said he wrote many of his best songs, including Hello, Mary Lou, in his candy apple red 1935 Ford coupe, parked near a Rockville reservoir. He is survived by his wife and three sons.