He was a key government official during the administration of president Ramon Magsaysay, best known for his stint as the head of the Presidential Complaints and Action Commission.

He finished high school in 1933 at the Ateneo de Manila, and acquired his bachelor of arts degree in 1937 at the same institution.

[1] During World War II, he was imprisoned in Fort Santiago by Japanese forces after he was suspected of being in contact with the Philippine Army.

He shared a crowded cell along with other prominent Filipinos such as Eulogio Rodriguez, Colonel Jose Olivares, and Bienvenido de la Paz.

[2] In 1945, he also served as the representative of the Philippine press at the United Nations Conference on International Organization in San Francisco.

[4] He also played a key role in four months of negotiations and the surrender of Luis Taruc, the leader of the communist group Hukbalahap, on 17 May 1954.

With the presidential election scheduled to be held in the same year, Magsaysay's death also triggered a chaotic political situation in the country, most notably with the ruling Nacionalista Party.

Manahan, among other Nacionalistas closely associated with Magsaysay, were disappointed by the "cold treatment" given to them by allies of President Garcia and decided to leave the party.

Despite having less money compared to the candidates of the two major parties, he was able to rally significant support among the masses with his popular appeal and his resemblance, both physically and ideologically, to the late President Magsaysay.

In the 1961 general election, the Grand Alliance joined forces with the Liberal Party to oppose President Garcia's bid for reelection and endorse Vice-President Macapagal's presidential candidacy.

[4] After President Diosdado Macapagal failed to meet the expectations of the Grand Alliance, several members, including Manahan, separated from the Liberal Party.

Prior to the declaration of martial law in 1972, Manahan served as the executive vice-president of the Manila Times Publishing Corporation.