An openly transgender woman is set to be executed for the first time in US history.

Unless Missouri governor Mike Parson grants clemency, Amber McLaughlin will die by lethal injection for killing a former girlfriend in 2003.

Before transitioning, the 49-year-old was in a relationship with girlfriend Beverly Guenther.

But when it turned sour, according to court records, McLaughlin began stalking Ms Guenther going to the office where she worked and sometimes hiding inside, killing her in November 2003.

She was convicted of first-degree murder in 2006 and sentenced to death.

A court in 2016 ordered a new sentencing hearing, but a federal appeals court panel reinstated the death penalty in 2021.

A clemency request on behalf of McLaughlin focuses on several issues, including her traumatic childhood and mental health issues, which the jury never heard in her trial.

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A foster parent rubbed faeces in her face when she was a toddler and her adoptive father used a stun gun on her, according to the petition.

It says she suffers from depression and attempted suicide multiple times.

The petition also includes reports citing a diagnosis of gender dysphoria, a condition that causes anguish and other symptoms as a result of a disparity between a person’s gender identity and their assigned sex at birth.

McLaughlin’s lawyer Larry Komp said: “We think Amber has demonstrated incredible courage because I can tell you there’s a lot of hate when it comes to that issue.”

But, he added, McLaughlin’s sexual identity was “not the main focus” of the clemency request.

There is no known case of a transgender inmate being executed in the US before, according to the anti-execution Death Penalty Information Center.

A friend in prison, Jessica Hicklin, says she saw McLaughlin’s personality blossom during her gender transition.

Though imprisoned together for around a decade, she said McLaughlin was so shy they rarely interacted. But as McLaughlin began transitioning about three years ago, she turned to Hicklin for guidance on issues such as mental health counselling and getting help to ensure her safety inside the male-dominated maximum-security prison.

“Definitely a vulnerable person,” Hicklin said. “Definitely afraid of being assaulted or victimised, which is more common for trans folks in Department of Corrections.”

The only woman ever executed in Missouri was Bonnie Heady, put to death in 1953, for kidnapping and killing a six-year-old boy.

Heady was executed in a gas chamber, side by side with the other kidnapper and killer, Carl Austin Hall.

Nationally, 18 people were executed in 2022, including two in Missouri.

Kevin Johnson, 37, was put to death for the ambush killing of a Missouri police officer. Carman Deck was executed in May for killing James and Zelma Long during a robbery at their home in De Soto, Missouri.

Another Missouri inmate, Leonard Taylor, is scheduled to die in February for killing his girlfriend and her three young children.