Lawyer, Amanda Telfer, 43, was crushed to death in August 2012 in Mayfair when the large unsecured frames toppled over.
Kelvin Adsett, 64, of Slough, Berkshire was convicted at the Old Bailey on Thursday of manslaughter by gross negligence and safety offences.
Adsett was the on-site project manager for IS Europe Ltd.
Damian Lakin-Hall, 50, of Cobham, Surrey was convicted of safety offences. He was acquitted of manslaughter.
IS Europe Ltd of Slough, Berkshire was convicted of safety offences.
The convictions follow an investigation by the Met’s Homicide and Major Crime Command with the assistance of the Health and Safety Executive.
Adsett and Lakin-Hall were bailed to appear at the same court for sentencing on 5 May.
Detective Chief Inspector Andrew Chalmers said: “The individuals and company who were convicted in this tragic case had a laissez-faire attitude to health and safety and did not take their obligations seriously.
“Each had a responsibility for the safety of the construction site but failed to deal with a basic task that very obviously then presented a serious hazard.
“Amanda died four-and-a-half years ago and this has been an incredibly long and complex case to bring before the courts with many many hours of enquiries carried out by my team.
“Her death was completely avoidable and it is satisfying for all involved in this case – and especially Amanda’s family – that the jury have convicted these people and companies today.
“Prosecutions such as this are so important in enforcing adherence to health and safety laws. This tragic case proves just why employers and employees should take their obligations to safeguard workers and the public seriously.”
Telfer, a freelance intellectual property and media lawyer, was walking along the pavement past a construction site in Hanover Square, W1 when three large unglazed window frames, together weighing 655kg, fell on her.
She died at the scene from massive crush injuries.
The window frames – one around 3.2m square and two approximately 3.3m x 1.8m – had been delivered the previous day as scheduled but couldn’t be fitted immediately due to other delays on site.
They were left on the pavement overnight, leaning against the building. No efforts were made to secure them and no barrier placed around them.
No checks were made on them when the individual defendants arrived on site the next morning.
As Telfer walked past, it is believed a door in the building blew open in the wind, hitting the frames and causing them to topple.
A worker inside tried to grab them but they fell, crushing her underneath.
Lakin-Hall told officers at the scene the frames had been secured to the wall with a ratchet strap – evidence showed that had never been the case.
Barry and Ann Telfer, Amanda’s parents, said following the verdict:
“Amanda was a bright lovely professional woman living her life to the full and making plans for the future. Her future was taken from her when she was crushed to death by half ton window frames which took two seconds to fall on her.
“The frames had been left standing, almost vertically, at the side of a public pavement, unsecured to anything, unattended and with no safety barriers around them.
“If construction companies and the people who work for them are not held to account for such high levels of negligence and incompetence then none of us is safe walking the streets next to construction sites. The Health and Safety training being given is totally inadequate, if risk of death to passers-by is ignored.”