He served as editor of the vernacular section of the Diariong Tagalog (Tagalog Newspaper), the first Philippine bilingual newspaper, in 1882. From 1890 to around 1895, he edited and published the newspaper La Solidaridad (Solidarity), mainly through his 150 essays and 66 editorials published under the nom de plume Plaridel.

Del Pilar's militant and progressive outlook was derived from the classic enlightenment tradition of the French philosophes and the scientific empiricism of the European bourgeoisie. Part of this outlook was transmitted by freemasonry, to which del Pilar subscribed.

Marcelo Hilario del Pilar y Gatmaitan was born in sitio Cupang, Bulacan, Bulacan, on August 30, 1850, to Don Julián Hilario del Pilar, a three time gobernadorcillo and a poet-grammarian, and Doña Blasa Gatmaitan. He was the last child and the fifth son among the ten children and was baptized Marcelo Hilario. Because there were many children in the family, Marcelo gave up his share of his inheritance for his other brothers and sisters. His elder brother, Fr. Toribio del Pilar, was exiled to Guam for his involvement in the 1872 Cavite Mutiny. The family adapted the surname Del Pilar in 1849 pursuant on the decree issued by Governor-General Narciso Claveria. Del Pilar was descended from the illustrious lineage of Gatmaitan, one of the sons of the pre-colonial ruling families of Bulacan and Pampanga.

He learned his first letters from his paternal uncle Alejo in 1860. Because his family was highly cultured, it was not long before he played the piano, violin, and flute. In Manila he took a Latin course in the school of José A. Flores in 1872 and then transferred at the Colegio de San José, where he finished his Bachelor of Arts degree. He also studied at the University of Santo Tomas, where he obtained his law degree. Reliable information on his years as a student is incomplete, though he clearly dropped out of law school a couple times, finishing degree only in 1881. This was due to a dispute with a parish priest of San Miguel, Manila in 1869 who was charging an exorbitant baptismal fee.

As a student, he favored overthrowing the Spanish government. Often, he met with his classmates like Mariano Ponce, Numeriano Adriano, Pedro Serrano Laktaw, and Apolinario Mabini in his Binondo house, and expounded on the need to peacefully fight Spanish rule. His mastery of Spanish language would help hasten development led him to teach Spanish to children in his neighborhood while he was a boarder of Mariano Sevilla, a Filipino secular priest. Then about the time of Cavite Mutiny, he used to meet regularly in a goods store in Manila with liberal Spanish creoles, mestizos, and Filipino intellectuals by whom he was politically indoctrinated about the affairs of the country. Fortunately, suspicion was not turned on him and he escaped prosecution in 1872.

He worked as oficial de mesa in Pampanga and Quiapo in January 1878. He worked for the Manila Royal Audiencia and at the same time he spread nationalist and anti-friar ideas in Manila and in towns and barrios of Bulacan. He married his second cousin Marciana (Tsanay) in February 1878. They had seven children and five died of infancy. Already a family man, he finally obtained his licentiate in jurisprudence in 1880.

Driven by his sense of justice and his own bad experiences with the clergy, del Pilar denounced in his publications on the violations of the clergy, the narrow-mindedness and hypocrisy. He made speeches in an open crowd, whether a cockpit, tienda, and town plaza. He delivered his tirades against the friars during fiestas, parties and funeral wakes. He wrote poems and essays defending Filipino interests and fought for the equality of Filipinos and Spaniards in his book "La Soberania Monacal en Filipinas" (Monastic Sovereignty in the Philippines).

Published in 1888 under his pseudonym, Plaridel, he underscores the failure of government in making good the noble promises of the first encounter between Spain and the Philippines, formalized by an agreement sealed with the blood of Legazpi and Sikatuna. The shortcomings of government are capitalized on by the friars to secure their temporal powers, promote their material interests and perpetuate themselves as the sole arbiters of spiritual and earthly matters in the Philippines; in effect, establishing a monastic rule.

On August 1, 1882, del Pilar founded the newspaper Diariong Tagalog (Tagalog Newspaper), with the help of Francisco Calvo y Muñoz, a wealthy Spanish liberal. This newspaper was the first to publish ideas for reforms in the Philippines. Editing its Tagalog section, he published among others, his nationalist and reformist articles, like the Tagalog translation of Rizal's El amor patrio. When lack of funds forced him to stop publication on October 31, 1882, he produced pamphlets making fun of friars, such as Dasalan at Tocsohan (Prayerbook and Teasing Game), Pasióng Dapat Ipag-alab nang Puso nang Tauong Babasa (Passion That Should Inflame the Heart of the Reader), Caiingat Cayo (Be Careful), Cadacilaan ng Dios (God's Goodness), Sagot ng España sa Hibik ng Pilipinas (Spain's Reply to the Complains of Filipinos), Dudas (Doubts) and La Frailocracia Filipina (Frailocracy in the Philippines). All were published in Madrid in 1889.

His parodies of the Our Father, the Hail Mary, the Apostle's Creed, the Ten Commandments and the catechism published in pamphlets which simulated the format and size of the novenas were highly effective propaganda. Unlike Rizal, who wrote his novels in Spanish, del Pilar wrote his propaganda pamphlets in simple Tagalog.