The new company, to be called Zip, will be a joint venture between the two directors, who will share the managing director title.
Morahan will manage the production house on a day-to-day basis, which will operate from The Paul Weiland Film Company's Newburgh Street office in London, with full administrative backup and resource support.
He will still be partnered by the producer Amy Appleton, with whom he teamed up at Harry Nash.
Zip's aim is to sign unknown directors with potential who will work on commercials, short films and feature-length productions. It has already signed one director, Joachim Trier.
Morahan said: "I had a fantastic time at Harry Nash, but I wanted to have a go at doing it myself. It's a great way to keep directing, but I'm at the stage when I want to do more, and get into different things."
The Paul Weiland Film Company's managing director, Mary Francis, added: "We are thrilled to welcome him back into what will be a very exciting venture."
Morahan left The Paul Weiland Film Company after two years in September 2000 for Harry Nash.
While there he directed ads for Rimmel, Gold Blend and Kellogg.