Fagg was born Pieter Fagg in Vlissingen in the Kingdom of the Netherlands on January 14, 1837, son of Captain Johannes Gerardus Fagg and Sara Jacoba Smith, and grandson of John Fagg and Jacoba Johanna van der Tollen, with John having Scotch and English ancestors. The captain was a ship owner, sailor, and merchant who owned a large grocery business in Vlissengen. His mother, Sarah Jacoba Smith, was a Vlissingen native from a prominent local family of English and Dutch ancestry. Captain Fagg never learned the Dutch language, speaking only English. Captain Johannes Fagg died at the age of 34, and for two years his wife managed the business. She married for a second time on March 5, 1841,Frederick T. Zetteler, a son of the royal tailor. A few years after Sarah's second marriage, and after the second bankruptcy of her husband the whole family came to the United States, taking passage at Antwerp on a sailing ship that landed them at New York City. They proceeded to Albany, New York and thence by canals and the Great Lakes to Milwaukee, landing July 3, 1848, and there they settled down on a farm. In 1853 Zetteler moved the family to Madison and opened a general store. The mother and children ran the store and the father worked for official offices of the State, including the Secretary of State of Wisconsin and the State Register of Deeds, and Peter worked in the office of former Governor Leonard J. Farwell. In 1858, a fire destroyed all of the family's property, and Zetteler returned to Milwaukee and went into the real estate business. Peter married Tyistke Tillema on May 3, 1859. They moved to Alto in Fond du Lac County in 1861, where he worked as a retail clerk in a general store. Fagg was elected a justice of the peace in Alto, and county supervisor for the same place, in 1862, and re-elected. In 1863 his stepfather was elected to the State Assembly back in Milwaukee. Fagg was appointed as a prison guard in the Wisconsin State Prison in 1865; then returned to Milwaukee in 1867, where he was appointed a police officer under Chief William Beck. He became a deputy sheriff under Sheriffs Parsons and McDonald, and resigned in October, 1873. He worked as a notary public and debt collector.
Assembly
In 1874 he was elected to succeed fellow Reformer Joseph Hamilton under the label of "Democratic Reform." He won with 995 votes, to 451 for Thomas Armstrong, who ran as an "Independent Republican". He was re-elected the next year as an "Independent Democrat", with 763 votes to 534 for regular Democrat George Tyre, serving with his stepfather Frederick Zetteler, who had just been elected once again to the Assembly from Milwaukee as a Democrat. Due to the division of the Assembly, no single party could assemble a majority. Fagg was persuaded to support Republican Sam Fifield for Speaker of the House. Fagg switched his vote and his influence, thus enabling the Republicans to control the Assembly. From that time on, he supported the Republican party cause, especially among his fellow Dutch Americans. Fagg was not a candidate for re-election in 1876, and was succeeded by Hamilton.