The case against West Mercia probationary constable Mary Ellen Bettley-Smith, 31, ended in a hung jury a day after Pc Benjamin Monk was convicted of the ex-footballer's manslaughter and told he will face an "inevitable" prison term.
Jurors deliberated for more than 21 hours at Birmingham Crown Court before telling the trial judge they would be unable, even if given further time, to reach a verdict on a charge alleging Bettley-Smith used unlawful force.
She told her trial she used necessary force to defend herself and others, but prosecutors claimed she had exaggerated the threat posed by former Aston Villa, Sheffield Wednesday and Ipswich Town star Mr Atkinson to "help" Monk's attempt to justify excessive force.
The Crown was granted seven days to decide whether to seek a re-trial. Bettley-Smith was given unconditional bail until Monday, when Monk is expected to be sentenced.
A six-week trial was told Monk, 43, fired a Taser three times - including a single 33-second discharge - and left two bootlace prints on Atkinson's forehead near his father's home in Meadow Close, Telford, in August 2016.
Jurors took 18 hours and 48 minutes to reach unanimous verdicts on Monk, who is believed to be the first police officer since 1986 to be found guilty of unlawful killing over a death in custody or following police contact in England and Wales.
Describing the outcome as an "historic trial" in a statement through the deaths in custody charity Inquest, Mr Atkinson's family said it was relieved that everyone now knew the truth of how he died.
They also called for improvements to the system for prosecutions involving police defendants to prevent delays in future.
The Assistant Chief Constable of West Mercia Police has said Monk's actions have damaged trust in the force, which is now undertaking a fast-track misconduct process.