[1][2] A member of the Democratic Party, McKiernan was elected to the state house in 2014.
[1] McKiernan attempted to run for re-election in 2020, but lost in a three-way primary to David Morales.
[1] He was elected by primarying incumbent representative Maria Cimini with support from house speaker Nicholas Mattiello, who supported McKiernan because Cimini did not support Mattiello's bid to become house speaker.
[3] Throughout his period in the state house, McKiernan garnered a reputation for being "ordinary", voting along party lines and rarely sponsoring legislation.
[4] In 2020, McKiernan faced two primary challengers, David Morales, a progressive who had support from the Democratic Socialists of America and the Sunrise Movement, and Angel Subervi, a centrist who primaried McKiernan due to the latter being perceived as out of touch with his district.