He will be one of five principals at the Wilton, Conn.-based independent agency.
A successor at Momentum could be named this week.
Mr. Stone, 44, had been Momentum CEO since April 2001. McCann-Erickson WorldGroup's O5, created this year to specialize in Olympic marketing, also reported to Mr. Stone.
Velocity said Mr. Stone will lead efforts in marketing and promotional areas related to TV packaging as well as property and event consulting.