Westall was a partner at Leonardo and Sumption was the interaction director. Earlier this year, the advertising agency acquired 100% of Hard Reality, its award-winning digital consultancy, which it then merged with existing direct shop Leonardo.
Burnett bought a minority stake in Hard Reality in November 1998, but moved to take full control of the agency in order to offer digital and direct services under one roof.
Sumption said, "We've been storing up personal projects for a long while and it is time to broaden our horizons a bit by moving into a much wider range than just marketing."
Westall said, "I had a great time at Leo Burnett, working with great people and great clients. I learnt a lot but feel that it is a good time to move on and pursue more innovative and creative projects."
Steve Barton, Leonardo CEO, said, "I think there is a lot of talent in this agency and people are stepping up into the roles that were held by Mark and Dan."
He said that the mergers of the two Leo Burnett units in January had been driven by the needs of clients, which wanted to have a direct, digital and data brief responded to at once, rather than three times.
Leonardo employs 30 staff and its clients include Morgan Stanley, Kelloggs and VSO.