Watts, who joined from 4Creative, replaced David Buonaguidi who left last April to pursue his own venture. Buonaguidi had been in the role less than two years.
Ramsey has been appointed on an interim basis.
Watts joined 4Creative as head of creative in August 2016 after 18 months as creative director at CHI & Partners (now known as The & Partnership London). Before that, Watts spent nine years at Fallon.
In a statement CP&B said: "CP+B London and Dan Watts have mutually agreed to part ways. We thank Dan for his time with us and wish him nothing but the best in his next ventures. While the search is on for a long-term replacement, we’re excited to have the excellent Russell Ramsey join us this week as our interim ECD."
Ramsey spent 11 years at JWT before standing down at the beginning of 2017.
The appointment comes after a torrid time for the beleaguered MDC Partners-owned shop.
Last year it lost its chief executive, Richard Pinder, creative chief and director of operations Arjun Singh, leaving it largely leaderless.
Helen Watts was drafted in as managing director from Bartle Bogle Hegarty London.
CP&B scored two out of nine in its Campaign School Report this year.