This was written by the great Gene Pitney. It was first recorded by country singer Johnny Duncan but doesn’t seem to have made any impression on the charts.
In 1961 it was released by Ricky (later Rick) Nelson. It got to No 9 in the States and No 2 in Britain.
Pitney recorded it himself for his 1962 debut album The Many Sides of Gene Pitney.
After the release of Hello Mary Lou, a similarity was noticed between it and an earlier song, Merry, Merry Lou, written by Cayet Mangiaracina and recorded by his band, the Sparks, in 1957.
His publisher sued for plagiarism and a settlement was reached, with Mangiaracina named as co-writer of Hello Mary Lou.