Passmore was elected to represent Risca east on Caerphilly County Borough Council in 2004, defeating incumbent Plaid Cymru councillor Harry Styles.

In July 2015, Passmore was selected as the Welsh Labour and Co-operative Party candidate for the Islwyn constituency of the Senedd.

[10] Passmore was briefly suspended from the Labour group in the Senedd on 26 June 2024 after she was alleged to have used several number plates on her car, which is a criminal offence.

The suspension was lifted in under 24 hours after a police investigation ruled that there was no evidence for this claim and that no offence had been committed.

[11] In October 2017 she was arrested by South Wales Police after failing to provide a breath test; as a result she was banned from driving for 20 months, fined £1,000, £100 victim surcharge and £620 in costs.