Senator Rosales was born on December 20, 1907 in Calbago, Calbayog City to Basilio Rosales and Aqueda Ras.
He finished Associate of Arts degree in 1920 and Bachelor of Laws degree both from the University of the Philippines.
In 1941, he was enmeshed in politics and for his political debut, he was pitted against the then Secretary of Public Works, Jose Avelino, who during the time, was Samar’s recognized political Kingpin. Hardly Senator Rosales assumed the position of Representative of the first district Samar and the dim of 1941 elections abated when Pearl Harbor went up in smoke and so, Senator Rosales went back to Visayas and join the resistance movement in Cebu to take charge of the Civil Affairs of the province.
In 1947, he ran against the son of his political enemy for the governorship of Samar, but the glamour of the Roxas’ bandwagon and the illusions it created in the minds of the newly-liberated Philippines proved too much for the aging Osmeña. With the rest of the Osmeña ticket Rosales lost his bid, only to stage a successful comeback when in 1951, he bested Baltazar Avelino in a return match for the gubernatorial seat of Samar. Once again he asserted his leadership, but it was bitterly achieved. For shortly before Senator Decoroso was persecuted by his political detractors. He was incarcerated and denied bail... for defamation! In the face of all these, Decoroso’s mother had a stroke. She died - her frail heart having broken down with misery and woe.
In 1953, he was elected as the Chairman of the League of Governors and the City Mayors of the Philippines.
In 1955, he was elected Senator for only a couple of years but he was member of ten standing committees such as the Provincial and Municipal Government and Chartered Cities.
He was a delegate to the 1971 Constitutional Convention and headed the Committee on Preamble, National Territory and Declaration of Principles of the 1986 Constitutional Commission.
He was married to Rosita Sepulveda with whom he had two children.