She frequently collaborates with Armando Iannucci and Steve Coogan, having played multiple characters in The Day Today, Brass Eye and Alan Partridge, and has also appeared in Toast of London and Two Doors Down.

Mackichan was born on 7 August 1962 in London and later moved with her family to the Scottish village of Upper Largo.

On stage, she appeared with Matt Di Angelo and David Haig in the Joe Orton black farce Loot.

In July 2011, Mackichan performed alongside Julian Barratt in Nikolai Gogol's comedy The Government Inspector at the Young Vic Theatre, London.

She has also voiced characters in several animated series including Bob and Margaret, Stressed Eric, Don't Eat the Neighbours and Bromwell High.

She played Cathy in the BBC comedy Two Doors Down from 2013 until 2022, when she decided to quit the show to concentrate on other acting roles.

In 2017 she played Feste in the Royal National Theatre's production of Twelfth Night and television film Death on the Tyne in 2018.

Mackichan appeared as the Archangel Michael in the Amazon Prime/BBC mini-series Good Omens and as Sarah in Channel 4 comedy-drama Pure in 2019.