The single-vehicle incident occurred on Battery Road at approximately 10.40pm on Friday night.
The male driver, who is in his late teens, was initially taken to Regional Hospital Mullingar after suffering serious injuries. He was later transferred to Beaumont Hospital in Dublin, where he remains in a critical condition.
A teenage girl who was travelling as a passenger on the e-scooter was also taken to Regional Hospital Mullingar. Her injuries are understood to be non-life-threatening.
Gardaí and emergency services attended the scene following the collision. Battery Road remains closed while Garda Forensic Collision Investigators carry out a technical examination, with diversions currently operating in the area.
Gardaí are now asking anyone who witnessed the incident to contact investigators.
They are particularly interested in hearing from motorists or other road users who were in the Battery Road area between 10.15pm and 10.45pm on Friday and may have captured relevant footage on a dashcam or other camera.
Anyone who has information is asked to contact Longford Garda Station on (043) 335 0570, the Garda Confidential Line on 1800 666 111, or any Garda station.
