Vice Mayor of Navotas (1998; 1999–2000)

Tiangco first sat as the vice mayor of the then-municipality of Navotas in early 1998. On June 30, 1998, Tiangco ascended as mayor due to an electoral protest against Mayor Cipriano Bautista but became vice-mayor again on February 1, 1999.

Mayor of Navotas (2000–2010; 2019–2022)

On May 12, 2000, Mayor Cipriano Bautista died, resulting in Tiangco's second accession as mayor until June 30, 2001. Tiangco ran for a fresh term with running mate Lutgardo Cruz, but Patrick Joseph Javier, an independent, won the vice-mayoral seat.

In 2004, Navotas was awarded as the “Cleanest Municipality of NCR”, and Tiangco remained mayor of Navotas until 2010. He also founded Partido Navoteño in the same year. He focused on the issues of garbage, and the beatification of Navotas City Hall. In 2007, Navotas was converted into a city, resulting in Tiangco becoming the first City Mayor. His brother John Rey succeeded him following the 2010 elections.

Being barred for reelection in congress, he ran for Mayor of Navotas in 2019. He was successful, defeating 2nd district councilor Dan Israel Ang by a wide margin. Tiangco succeeded his younger brother, John Rey, who in turn took his vacated seat as representative for the lone district of Navotas. He was in-charge of the Navotas city government during the COVID-19 pandemic. He donated his salary from April 2020 to the end of his term in June 2022 worth almost ₱3.3 million to families who are not part of the national government's emergency subsidy program.^[3]

Congressman (2010–2019; 2022–present)

Tiangco ran for Congress unopposed and assumed office on June 30, 2010, as the first representative of Navotas's at-large congressional district. As congressman, he gave scholarships to 900 Navoteños, and ensured free medical treatment major hospitals with his Medical Assistance Ordinance. He also initiated subsidised housing projects in Barangay Tanza.

In 2012, Tiangco served as a state witness at the impeachment trial of Supreme Court Chief Justice Renato Corona. Tiangco joined United Nationalist Alliance (UNA) and later became party president. In 2013, he sought for a second term and won with 70.05% of votes, defeating Rico de Guzmán, a former balút vendor.

Tiangco ran for a third and final term in 2016 and won. In July 2016, Tiangco left and resigned as party president of UNA over a House minority leadership row and became part of the independent minority in the House of Representatives until his term ended in 2019.^[4]

Tiangco once again ran for Congressman and switched places with his brother John Rey in 2022 and elected in a landslide victory.