His arrival follows a period of transition for the charity, with its chief executive Andrew Flanagan to leave in March.
Farthing was drafted in after interim fundraising director Mike Parker left in November. Parker had replaced Paul Amadi, who departed a year ago to become director of funding at Diabetes UK.
The NSPCC has been flooded with calls since the Jimmy Savile scandal broke last year. When the report of Operation Yewtree, the inquiry into the allegations, was published last week, the charity said the late media personality was one of the most prolific sex offenders in its history.
The NSPCC launched a national print campaign last month to ensure revelations about Savile were not seen as a purely historical problem. It encouraged people to report any concerns of children currently being abused.