He spent his entire political career from 1991 to 2015 as a city councillor in the Basque capital of Vitoria-Gasteiz, serving as party leader from 2003, and mayor from 2007 to 2011.

He was elected to the city hall of Vitoria-Gasteiz in 1991, serving in the local government under mayor José Ángel Cuerda of the Basque Nationalist Party (EAJ-PNV).

[1][2] In the 2007 local elections, the PSE-EE received the most votes and Lazcoz was installed as the first Socialist mayor in the city's history.

[1] In October 2010, Vitoria-Gasteiz was given the European Green Capital Award for 2012 ahead of more favoured bids such as that of Barcelona.

[4][5] In October 2014, he confirmed that he would not run in 2015, in order to leave politics to the next generation; he also left his internal roles in the PSE-EE in the province of Álava.