Humphreys is from Ringsend. He was first politically active in the campaign against the Eighth Amendment in 1983, which gave constitutional recognition to the right to life of the unborn.
Humphreys first ran for election at the 1999 local elections, when he was elected as a member of Dublin City Council, representing the South East Inner City local electoral area. He represented the area until his election to the Dáil in 2011.[3] He was re-elected in 2004 and 2009, topping the poll both times. During this period, Humphreys worked with then Lord Mayor, Andrew Montague to deliver the Dublin Bikes scheme.