She was engaged to respected documentary filmmaker Chris Terrill in 1999, after only 12 days in Cambodia, where they were making a film about landmines. Mills ended their relationship five days before their planned wedding day, later telling friends in the media that she had called the wedding off because Terrill was gay, an MI6 agent, and that his mission was to undermine her anti-landmine work.
Terrill had once told Mills that he had been interviewed by the intelligence services when he was thinking of a career with the Foreign Office, but later said, "I soon realised that Heather had a somewhat elastic relationship with the truth, which she was able to stretch impressively sometimes". Terrill also claims that although Mills stated she was a vegetarian at the time, she often cooked her speciality dish for him; Lancashire hotpot (which contains lamb) and her ex sister-in-law, Dianna Karmal, claims that Mills only became a vegetarian after meeting McCartney.