Tom Conniffe, who was also born in the borough but now lives in north London, will take part in Walk 101, which will see him trekking along the South Downs Way from Winchester to Eastbourne in three days, from June 18-20.
In doing so, he will raise money for The Brain Tumour Charity, Pancreatic Cancer UK and Stroke Association.
The 43-year-old policy officer, who has worked for the council for 12 years, hopes to raise £5,000 for the good causes.
He said: “I lost my aunt Chrissie on Christmas Eve following a stroke and her brother, my uncle Paddy, is living with the effects of one he suffered early in 2014.
“Ian, a friend from university, died aged just 41 in November from a brain tumour, leaving behind a wife and two young children.
“And my grandfather died of pancreatic cancer and Joan, the mother-of-two dear friends from my school days, is currently battling the disease.
“So all three causes have a huge personal resonance and I want to do my bit.”
To sponsor Tom, go to http://tinyurl.com/o8c48eo.