At the age of 30, Marcela Coronel Mariño married Felipe Encarnacion Agoncillo, a Filipino lawyer, and a jurist, and gave birth to six children.

When her husband was exiled in Hong Kong during the outbreak of the Philippine Revolution, Marcela Mariño Agoncillo and the rest of the family joined him and temporarily resided there to avoid the anti-Filipino hostilities of the occupying Spain.

While in Hong Kong, General Emilio Aguinaldo requested her to sew the flag that would represent the Republic of the Philippines.

Doña Marcela Mariño de Agoncillo, with her eldest daughter Lorenza and a friend Delfina Herbosa de Natividad, niece of Dr. Jose Rizal, manually sewed the flag in accordance with General Emilio Aguinaldo's design which later became the official flag of the Republic of the Philippines.

The convent she was studying in was the Santa Catalina College of the Dominican nuns, an exclusive school for girls,[11] established in the Walled City of Intramuros where she finished her elementary and secondary education.

[17] After learning of the plans of the Governor-General of the Philippines (Basilio Augustin y Davila) to deport Don Felipe Encarnacion Agoncillo, he sailed to Yokohama, Japan, staying there only briefly until proceeding to Hong Kong where he joined other Filipino exiles who found asylum when the revolution broke out in 1896.

Also, Josephine Bracken, Jose Rizal's fiancée, sought refuge in their house when the Spanish authorities threatened to torture her.

[11] After the signing of the Pact of Biak-na-Bato on December 14, 1897, General Emilio Aguinaldo visited the Agoncillo residence in Hong Kong after their voluntary exile.

[22] After having met them, General Emilio Aguinaldo requested that Doña Marcela Mariño de Agoncillo immediately hand-sew a flag according to his design which would embody the national aspirations of all Filipinos.

Made from 100% fine silk which she bought in Hong Kong,[26] the flag was embroidered in gold and contained stripes of blue and red and a white triangle with the sun and three stars on it.

[24] On May 17, 1898, the flag was delivered personally by Agoncillo[4] and was packed among the things President Emilio Aguinaldo brought back to Manila.

[29] In response to the message written by General Emilio Aguinaldo, Doña Marcela Mariño de Agoncillo wrote the following statement when she was interviewed: In the house at 535 Morrison Hill, where I lived with my family, exiled from our country on account of the national cause, I had the good fortune to make the first Philippine flag under the direction of an illust[r]ious leader Gen. Emilio Aguinaldo y Famy...It took me five days to make that National Flag, and when completed, I myself delivered it to Gen. Emilio Aguinaldo before boarding the transport McCullo[ug]h...Gen. Aguinaldo is the best witness who can give the information whether or not that flag was the first to be displayed in Cavite at the beginning of the revolutionary government against the government of Spain in these islands.Doña Marcela Mariño de Agoncillo and her daughters stayed in Hong Kong from 1895 to 1906.

Their funds had run out because of the heavy expenses incurred by Don Felipe for his diplomatic activities in France and in the United States.

After the death of Don Felipe, Agoncillo's remaining family suffered from starvation due to their meager supply of food, water and other needs.

Her remains were brought from Taal to Manila and interred alongside her husband in the Catholic cemetery of La Loma according to the wishes of her last will.

[35] Renowned Filipino painter Fernando Amorsolo painted the historical sewing and is nationally known as The Making of the Philippine flag.