In 1999, Heather, along with Greg Avery and Natasha Dellemagne, set up Stop Huntingdon Animal Cruelty (SHAC) with the aim of closing Huntingdon Life Sciences (HLS), a contract animal-testing company based in Cambridgeshire.

Nicholson was sentenced to 11 years imprisonment in January 2009 for conspiracy to blackmail in connection with the SHAC campaign. Nicholson pleaded not guilty to the charges.

She was released on February 11, 2012 after serving 5 and half years of her 11 year sentence.