Farthing will join the charity formed by the merger of Age Concern and Help the Aged in March, shortly before the organisation will become publicly known as Age UK.
Farthing has worked at Cancer Research UK since 2005 and previously worked for an agency that produced adverts for Age Concern and Help the Aged.
Tom Wright, chief executive of Age Concern and Help the Aged, said: "We are delighted to welcome Paul to the charity at this exciting time.
"His distinguished career spanning 20 years in fundraising and marketing will prove extremely valuable in helping us realise our ambitions."
The new organisation will have an annual turnover of more than £150m.