Juan B. Alegre was born on February 2, 1882, in Casiguran in the province of Sorsogon in the Bicol Peninsula.

Alegre was an abaca plantation owner[9] and reportedly one of the wealthiest citizens[10] of the Philippines of his time.

After completing his training in the Philippines, Juan B. Alegre attended Yale University from 1903 to 1905 but had to leave his 3Ls in Yale Law School, abetted by business concerns from the death of his father, Narciso Alegre Pellicer.

In 1922, he was elected to the Senate of the Philippines[17] for the Sixth Senatorial District[18] on behalf of the Nacionalista Party.

But before Alegre could take his place in the 9th Philippine Legislature, however, he died in his home in Manila after being sworn to office[20] at the age of 49,[21] on June 14, 1931[22] from complications of gastric ulcer.