Gillian Beckingham, 46, was charged with seven counts of manslaughter following the outbreak at the Forum 28 arts centre in Barrow, Cumbria, in July and August 2002.
On Thursday (21 April) the jury at Preston Crown Court found the married mother, from Grange-over-Sands, guilty of breaching health and safety laws.
But the jurors were discharged today after failing to reach verdicts on the manslaughter charges after 23 hours of deliberations over five days.
Barrow-in-Furness Borough Council, which owns Forum 28 and employs Beckingham, was cleared of corporate manslaughter midway through the trial, but pleaded guilty to breaching the Health and Safety at Work Act.
Beckingham's two-month trial followed the deaths of Richard Macauley, 88, Wendy Milburn, 56, Georgina Somerville, 54, Harriet Low, 74, Elizabeth Dixon, 80, June Miles, 56, and Christine Merewood, 55, all from Barrow.
A further 172 people were infected with the disease, which spread from the air-conditioning system at the Forum 28 arts centre, with nearly 500 people admitted to hospital suffering from respiratory problems. Many of the victims were left as invalids.
Beckingham showed no emotion as the jury foreman told Mr Justice Poole that they could not reach a verdict on the manslaughter charges.
The judge dismissed the jury and the prosecution said that no decision had yet been made on whether there would be a retrial. A date has yet to be set for the sentencing of Beckingham and the council over the health and safety breaches.
The court had heard that Beckingham, who was born in York and trained at the School of Architecture in Leeds, was head of the Design Services Group at Barrow council when the outbreak hit the town.
The prosecution claimed she was responsible for the upkeep of the 'ageing' air-conditioning system at Forum 28, but, just months before the outbreak, she allegedly cancelled the contract that ensured it was kept clean and safe.
Without such maintenance work, the system provided 'an almost perfect breeding ground for the legionella bacteria', the jury was told.