As legislator

Sarmiento served three terms as representative of the 4th District of Bulacan from 1992–2001. It was under that term that he authored Republic Act 8797, an act converting the Municipality of San Jose del Monte in the Province of Bulacan into a component city to be known as the City of San Jose del Monte. Three months before the end of his third and final term, he was appointed by Gloria Macapagal Arroyo as Presidential Adviser on Agricultural Modernization and concurrently appointed as Chairman of the National Food Authority, a position he held until 2004.

As city mayor

Sarmiento successfully vied for mayor of San Jose del Monte in 2004 but lost his re-election bid to then Representative and former two-term Municipal Mayor and the first City Mayor Eduardo V. Roquero (who ran under the Kampi banner) in 2007, amid allegations of cheating and bomb threats. In 2009, after a seemingly lengthy legal battle on the resolution of his election protest which affected even the residents of the city (the Comelec earlier ruled that Sarmiento is the duly-elected mayor, thus nullifying Roquero's election), he returned to his mayoralty seat after the Supreme Court dismissed the petition of Acting Mayor Reynaldo S. San Pedro to stop Sarmiento from assuming the seat. San Pedro became the Acting Mayor of the city when Mayor Eduardo V. Roquero died on August 24, 2009.^[2]

Sarmiento formally assumed control of City Hall on October 26, 2009, after San Pedro peacefully conceded his hold of the mayorship of the city, following the Supreme Court, Comelec and DILG decisions and the effectivity of the contested Comelec resolution. (Per the DILG order, San Pedro reverted to his old position of city vice mayor.)^[3]

He ran for a full term as city mayor on 2010 but lost to outgoing Vice Mayor Reynaldo San Pedro.